AU2013205904B2 - Full Strength Threaded Bar - Google Patents

Abstract

5175S-AU A full strength thread on a steel bar is disclosed. The bar has a ductile interior or core (132) and a thermally hardened exterior or shell (130). A thread (111) is formed on the exterior of the bar and the threaded portion is subsequently thermally treated to harden the thread but leave the interior of the bar still substantially ductile. The result is that the average strength of a transverse cross-section through the bar at a root of a thread is greater than or equal to the average strength of a traverse cross-section of the non-threaded remainder of the bar. Ac Ambient / 1 143 Time 110A Fig 28 110 Fg 3102 -ig3 F 035 13011 Fig 13ig3 5i.2 0 5 110 150 Fi 2 i 31g3 Fig 35

Description

Full Strength Threaded Bar

Field of the Invention

The present invention relates to a method of making a threaded bar for connection to other bars, coupling devices or construction elements. In particular the invention is for making a threaded bar wherein the strength of the threaded section is greater than or equal to the strength of the parent bar. More particularly the present invention relates to the threading of bars used for the reinforcement of concrete.

Background Art

Bars used in many engineering applications and in particular for the reinforcement of concrete are required to be joined together to transfer forces through engineering structures. The strength of a bar is dependent on the cross-sectional area at the minimum diameter of the bar. Engineering standards nominate minimum strengths of reinforcing bars based on the nominal cross-sectional area of reinforcing bars.

Modern construction standards require that the strength of the connection exceed the strength of the bars which are to be joined together. Minimum strength of reinforcing bars is not the only requirement, standards require reinforcing bars to be ductile. Australian Standards for deformed reinforcing bars for normal applications (N class) are required to provide a minimum ductility of 5% uniform elongation (strain after yield before reaching the maximum tensile strength) and E class bars for seismic applications must have a minimum of 10% uniform elongation to allow for controlled redistribution of loads at plastic hinge zones. It is essential therefore that methods for connection of reinforcing bars provide not only sufficient strength but preserve the ductility of the parent bar.

Methods for joining bars using a thread formed onto the end of each bar and screwed into internally threaded coupling devices were taught by US 1078007 (Stange 1913) for parallel threads and US 3,415,552 (Howlett 1968) for tapered threads.

Thread cutting is the simplest and cheapest method for threading a bar however the process reduces the cross-sectional area of the bar at the root of the thread which reduces the threaded section to less than the design strength of the bar. The design strength of the threaded bar must therefore be reduced proportionately, wasting material, energy and labour.

Reinforcing bars for concrete are specified (and designed) with mechanical properties for minimum specified strength and ductility based on the nominal cross-sectional area of the bar. Some manufacturing processes yield bars which have homogeneous cross-sectional microstructure with homogeneous strength and ductility. Other manufacturing methods result in bars which have averaged mechanical properties which meet the minimum strength and ductility but the microstructure and mechanical properties are non-homogeneous and vary across the cross-section of the bar. The surface strength and hardness of these non-homogeneous bars is much higher than the average strength and has lower ductility, but the interior steel in the core of the bar has much lower strength but higher ductility.

When threads are cut onto non-homogeneous bars, not only is the stressed cross-section itself reduced, but some or all of the strong material needed to provide the specified strength of the bar is removed, exposing the soft interior material at or near the root of the threads, i.e. at the critical stressed diameter. The loss of strength resulting from the removal of the strong material and the reduced cross-sectional area markedly reduces the average strength of the threaded section to below the design strength of the bar.

Known methods for making threaded bars equal in strength to the parent bar rely on increasing the bar diameter (and cross sectional area) or increasing the material strength in the threaded section by cold forming the material before or during threading, or both.

Known methods for increasing the diameter of the threaded section before threading are taught by EP0563490A (Bernard, 1992) and W09522422A1 (Carter, 1995). The bar end is first increased in diameter by cold forging (upsetting) the end of the bar over a short length and subsequently threading with a thread size larger than the nominal bar diameter. US 5,660,594 (Vilkakainen) teaches a method of hot forging to provide a longer thread length to overcome one drawback of the cold-upsetting process which limits the length of the thread.

Colarusso, US 7,507,048, teaches a method of increasing the strength of the end of a bar by cold working the steel. In this case the bar is first formed by flattening the ribs prior to the formation of a thread. By this means the strength of the cold-worked threaded section is increased above the strength of the parent bar and longer threads may be produced.

These prior art solutions introduce additional problems. Methods which rely on increasing the diameter of the bars result in larger diameter threads and coupling devices which make the placement of the bars and concrete more difficult during construction as a result of the increased congestion which results.

In recent years the strength of construction steels has been increased to improve construction efficiency and minimise waste. The average yield strength of modem reinforcing bars is now twice that of common “Mild” steel and has a proportionately lower ductility. Cold-working to increase the bar diameter or improve the steel strength may effectively compensate for the loss of strength of the reduced cross-sectional area in a thread but this risks reducing the ductility to below the specified minimum required by the bar specifications, risking brittle failure in or near the threaded section of the bar.

Robust design requires the maintenance of high ductility to control deformation under design load and overload conditions and is particularly important for seismic design.

The known methods for threading bars which rely on cold working are silent on the deleterious consequences of the reduction of ductility resulting from cold working. Cold upsetting bars to sufficiently large diameters to compensate for the loss of area from threading results in high surface strains, typically 15-20% in the upset area.

Surface strains exceeding the ductile limit result in cracking and brittle failure, a particular risk for bars with high strength (low ductility) surface structures. Likewise, cold working and swaging to improve the average material strength can lead to cracking of the high strength surface material and a consequent risk for brittle failure under service loading.

Not all steels are manufactured to be suitable for hot forging. Some are prone to hot cracking during forging and depending upon the chemical composition of the steel, the heat required for forging may result in cracking during cooling or reduce the strength of the bar in the adjacent heat affected zone.

None of the known methods for cold or hot working discuss the effects or applicability of these methods when used for making threads on bars of non-homogeneous microstructure and mechanical properties, typical of modern reinforcing bars used in concrete construction. Cold working can result in cracking of the low ductility high strength surface layer. The high temperatures 1000-1100°C and the length of time at this temperature during the heating and forging process causes the steel adjacent to the forged end to be heated above 500°C. This over-tempers and softens the high strength outer layer on which non-homogeneous bars depend for their strength, causing the average bar strength in the heat affected zone to fall below the specified strength.

To guarantee freedom from defects and minimise the risk of in-service failure, products made by these known methods must be inspected and proof load tested which increases manufacturing costs.

Genesis of the present Invention

The genesis of the present invention is a desire to provide a simple, reliable and economical method of producing a threaded end on a steel bar which is stronger than the parent bar and of similar ductility. The method should preferably be equally applicable to bars made with a homogeneous or non-homogeneous microstructure.

In particular the invention provides a method for manufacturing threaded bars from concrete reinforcing steel bars to enable them to be connected together or to other components or structural members without loss of design strength and ductility. The method is preferably able to economically produce threaded bars with standard metric threads of the same nominal diameter as the bar, using simple thread-cutting techniques.

Summary of the Invention

In accordance with a first aspect of the invention there is disclosed a method of forming a full strength thread on a steel bar of nominally circular transverse cross-section, said bar having a ductile interior and a thermally hardened exterior, said method comprising the steps of: (i) forming a thread having a plurality of turns at one end of said bar so as to remove some or all of said hardened exterior at said one end; and (ii) subsequently thermally heat treating said one end to thermally harden at least said thread but leave the interior of said bar inside said thread still substantially in its original ductile state, whereby the average strength of a transverse cross-section through said bar at a root of said thread is greater than or equal to the average strength of a transverse cross-section of the non-threaded remainder of said bar.

In accordance with a second aspect of the invention there is disclosed a modified threaded steel bar of nominally circular transverse cross-section having a ductile interior and a thermally hardened exterior, said bar being modified by (i) forming a thread having a plurality of turns at one end of said bar so as to remove some or all of said hardened exterior at said one end, and (ii) subsequently thermally heat treating said one end to thermally harden by quenching and tempering at least said thread but leave the interior of said bar inside said thread still substantially in its original ductile state, whereby the average strength of a transverse cross-section through said bar at a root of said thread is greater than or equal to the average strength of a transverse cross-section of the non-threaded remainder of said bar.

In accordance with a third aspect of the invention there is disclosed a full strength threaded bar comprising a steel bar of nominally circular transverse cross-section, having a thread having a plurality of turns formed at one hardened end thereof to remove some or all of the exterior material, and having a ductile interior and a thermally hardened exterior, wherein said thread is thermally heat treated after forming to thermally harden said thread by quenching and tempering whereby the longitudinal tensile strength of said one end is greater than or equal to the longitudinal tensile strength of the non-threaded remainder of said bar.

In a preferred embodiment of the invention, the threaded bar-end has a nominal thread diameter similar to, or less than, the nominal diameter of the parent bar, but has a higher strength than the parent bar to compensate for the loss of area and to ensure that failure of the threaded bar under load, always occurs in the parent bar, rather than in the threaded section. This provides certainty of design for designers, optimises efficiency and reduces waste.

After threading the bar-end, the threaded end is rapidly heated in a furnace (induction, gas or other) to a temperature exceeding the critical transformation temperature Ac (approximately 723°C) and immediately quenched (typically in a bath of agitated water) for a short time, sufficient to cause an outer layer of the steel containing the threads to harden but not for so long a time that the interior of the bar is appreciably hardened. The process is sufficiently fast so as not to reduce the strength of the parent bar adjacent to the heated zone, to less than the specified strength.

The bar is subsequently withdrawn and allowed to cool slowly, whereupon the heat from the hot, inner core material tempers the outer layer of hardened steel containing the threads which provides ductility and toughness to the threaded end.

The heating temperature, time and severity of quenching and air cooling are selected to ensure that the bar is not over-hardened but remains ductile and provides an average steel strength at the reduced cross section at the root of the thread which is sufficient to compensate for the loss of cross-sectional area resulting from the threading process.

The advantages of this method are that it may be adapted to any type of thread geometry including parallel and tapered threads which can be economically made using simple thread cutting processes and the mechanical properties of the threaded end may be restored to preserve the specified design properties of the bar.

Brief Description of the Drawings

Preferred embodiments of the present invention will now be described with reference to the drawings in which:

Fig. 1 is a side elevation of a prior art bar of no specific cross-sectional shape.

Fig. 2 is an axial cross section of a bar of Fig.1 showing a minimum cross sectional area described within a circumscribing circle.

Fig. 3 is a prior art threaded bar of Fig. 1 with a parallel thread cut onto one end of the bar,

Fig. 4 is a vertical section through the longitudinal axis of the threaded bar of Fig. 3, Fig. 5 is an axial cross section X-X at the thread root of the bar shown in Fig. 3 Fig. 6 is a side elevation of a prior art bar with a tapered thread on one end,

Fig. 7 is a vertical section through the longitudinal axis of the threaded bar of Fig 6

Fig. 8 corresponds to Fig. 3 showing a parallel threaded bar after failure

Fig. 9 corresponds to Fig. 6 showing a taper threaded bar after failure

Fig. 10 is a side elevation of a prior art bar with an upset end prior to threading

Fig. 11 is a side elevation of the bar of Fig. 10 after threading.

Fig. 12 is a vertical section through the longitudinal axis of the threaded bar of Fig 11 Fig. 13 is an axial cross-section at the thread root of the bar shown in Fig. 12 Fig. 14 corresponds to Fig. 11 showing the threaded bar after failure Fig. 15 is a side elevation of a typical concrete reinforcing bar

Fig. 16 depicts the steel microstructure at the axial cross section A-A shown in Fig 15. Fig. 17 is a side elevation of a parallel threaded reinforcing bar similar to Fig.3 Fig. 18 depicts the steel microstructure at section B-B at the thread root shown in Fig. 17.

Fig. 19 is a longitudinal cross-section of the bar shown in Fig 17

Fig. 20 is similar to Fig. 19 but with a tapered thread similar to Fig 6 on a bar of Fig. 15.

Fig. 21 is a side elevation of the preferred embodiment of the invention prior to thermal processing

Fig. 22A is an axial cross section B-B at the thread root of the bar shown in Fig. 21

Fig. 22B is a diagram of the metallurgical structure of Fig. 22 A

Fig. 23 is a side elevation of the heating process of the preferred embodiment.

Fig 24 depicts the heating zones at cross-section B-B during heating shown in Fig. 23 Fig. 25 is a side elevation of the quenching process following heating of Fig. 23.

Fig. 26 depicts the steel microstructure at section B-B after quenching

Fig. 27 is a side elevation of the preferred embodiment during and after final cooling to ambient temperature after quenching.

Fig. 28 is a diagram depicting the thermal processing steps shown in Fig 23-27.

Fig. 29 is a vertical cross-section showing the different strength zones after thermal processing of the preferred embodiment Fig 27 with parallel threaded end.

Fig. 30 is similar to Fig 29 but shows a second embodiment with a tapered threaded end.

Fig. 31 corresponds shows a threaded bar of the first embodiment after failure.

Fig. 32 shows a vertical section between prior art threaded bars of Fig. 11 when coupled with similar bars by a coupling device of the same strength.

Fig 33 shows an axial cross section of Fig. 32

Fig. 34 shows a vertical section of the preferred embodiment of the invention of Fig. 27 when coupled with similar bars by a coupling device of the same strength.

Fig. 35 shows an axial cross section of Fig. 34

Fig. 36 shows a vertical section through a concrete floor-wall joint with a bar of the first embodiment in the floor connected to an anchoring device in the wall.

Detailed description

Turning now to Figs. 1-20 showing prior art threaded bars, these take the form of a bar 1 with a distal end region 2 , a region 3 distant from the end 2, an outside surface 4 and a specified minimum cross-sectional area 5 wholly contained within the perimeter of surface 4 which is shown cross-hatched in Fig. 2. The nominal diameter of the bar is defined by the diameter 6 of a circumscribing circle 7 of equivalent area to area 5.

Figs. 3-5 show a first type of prior art threaded bar 10A with a parallel threaded section 11 formed at an end 2 of the bar 1 by machining or by thread rolling the surface 4 for a determined length 13. Figs. 4-5 shows that the distance 14 between the root 12 of the threaded section 11 defines a thread root diameter 14 of circumscribing circle 15 which is less than the nominal bar diameter 7. The cross sectional area 16 of the root circle 15 is shown cross-hatched in Fig.5 and is evidently less than the nominal cross-sectional area 5 of the bar 1.

Figs. 6 and 7 show a corresponding view and vertical cross-section of a second type of prior art threaded bar 10B which has an end region 2 in which the threaded section 17 is of a form which tapers from a maximum diameter 19 to a minimum diameter 18 and where the maximum diameter 19 is less than the minimum nominal diameter 6 of the bar 1.

It is evident that the maximum cross sectional areas of the threaded ends 11 and 17 defined by the diameters 14 and 19 corresponding to Fig. 4 and Fig. 7 respectively have a cross-sectional area less than the minimum area 5 of the bar 1.

The strength of the bar at any point along its length is is directly proportional to the material strength and the cross-sectional area at that point. Threaded prior art bars 10A and 10B, have lower cross-sectional area 16 than the nominal area 5 and proportionately lower strength in the threaded sections 11,17 than in the non-threaded sections 3 of the bar 1.

Figs. 8-9 shows typical failures after a breaking force has been applied between the threaded section 11, 17 of the end 2 of a bar 1 and the non-threaded bar region 3. Failure 19 occurs at a reduced cross-section at the root 12 in the threaded section 11 or 17 of the threaded bars 10A and 10B.

The design strength of these prior art threaded bars 10 is therefore less than the minimum design strength of the bar 1 based on its specified minimum cross-sectional area 5. As a result, more prior art threaded bars 10 must be used than non-threaded bars 1 for the same aggregate design strength, resulting in wasted materials, energy and labour.

Figs 10-14 show another type of prior art threaded bar 10C in which the threaded end 21 has a breaking strength which is greater than or equal to the strength of the bar 1. This is achieved by first enlarging (by upset forging or other means) an end region 20 of the end 2 of the bar 1 over a sufficient length to allow a thread 21 to be cut or rolled onto this enlarged region 20 with thread root 23 and a root circle diameter 24 greater than the nominal diameter 6 of the bar 1.

As shown in Figs. 12 and 13 the root circle diameter 24 is larger than the nominal bar diameter 6 and defines a circumscribing circle 25 with a cross-sectional area exceeding the minimum cross sectional area 5 shown as hatched in Fig. 13. Failure can therefore be expected to occur in the non-threaded section 3 of the threaded bar IOC rather than in the enlarged threaded section 21 as shown in Fig. 14.

It is evident that the thread 21 may be of either parallel shape shown in Fig. 11 or other form including tapered threads similar to but larger than shown in Fig. 6.

Fig 15 shows a prior art bar designated 1RB for reinforcing concrete made from steel with a non-homogeneous microstructure and mechanical properties and which is formed with intermittent ribs 26 displaced along its length. Fig 16 represents the cross-sectional microstructure of the steel at a horizontal section A-A at some point 3 along the bar 1RB. For simplicity this bar 1RB is shown to be nominally round with its surface 4 being the perimeter 7 of a bar 1 with a nominal diameter 6.

Fig. 16 shows the bar 1 has two different zones 31 and 32 indicated by different cross-hatching to reflect different microstructures and mechanical properties.

An exterior “shell” region 30 with an axial cross sectional area 31 lies adjacent to the surface 4 of the bar 1 and an interior “core” region 32 of axial cross-sectional area 33 lies beneath it. In Fig. 16 the broken line 34 represents a notional boundary at a radial shell depth 35 from the surface 4 which indicates the transition between the regions 30 and 32.

The steel shell 30 has a mechanical strength significantly higher and a ductility which is significantly lower than that of the underlying steel of the core 32.

The overall strength of the bar 1 is determined by the sum of the individual strengths of the shell 30 and the core 32, which are determined from their cross-sectional areas 31 and 32 respectively multiplied by the material strength of the each of the shell 30 and core 32.

This arrangement is best explained by an example of a bar 1 with a nominal diameter 6 of 20mm and minimum area 5 of 312mm2 which has a minimum breaking force of 202.8kN and minimum material strength of 650MPa.

If the notional transition boundary 34 lies at a shell depth 35 approximately 2mm below the surface, it defines a core 32 with a diameter of 16mm. Referring to the following Table 1 below, it can be seen that when the average strength of the steel in the shell area 31 is 1 lOOMPa and the strength of the steel in the core area 33 is 450MPa, the combined breaking force is 214.9kN which has an average strength of 689MPa over the full cross section 5 of the bar 1, which exceeds the minimum specified requirement of 650MPa.

Table 1

Figs. 17-20 show a prior art threaded bar 10A made from a reinforcing bar 1RB of the type shown in Figs 15-16 where the thread 11 is machined into the external surface 4 of the bar 1. Fig 18 shows the cross-sectional microstructure at section B-B at the root of a thread 12 shown in Fig. 17.

It is evident that the root circle 15 defined by the thread 11 not only reduces the nominal diameter 6 of the bar 1 but also removes high strength steel of the external shell 30 which reduces the cross sectional area 31 of the external shell 30 and the average strength of the threaded section 11.

The longitudinal cross-sectional diagram of Fig. 19 indicates the thinning effect of the removal of the material from the shell 30 in the threaded region 11 of a parallel threaded bar.

Fig. 16 shows that the reduction of the shell 30 thickness in a taper thread 17 completely exposes the weak core material 31 at the end of the bar 2 where the thread diameter 18 is least. As a result, the average strength of the threaded section 17 at the minimum diameter 18 has reduced to the low strength of the core.

By way of example, for a threaded bar 10A with the same properties in the unthreaded section 3 as shown in Table 1, the effect of the both the reduction of area of the steel and the removal of part of the shell 30 is clearly evident. In this example a bar 1 with a nominal diameter 6 of 20mm is machined with a parallel M20 metric coarse pitch thread 11 which has a root circle diameter 14 of 16.9mm. The strong shell area 31 is significantly reduced to an annulus between the transition boundary 34 with a diameter of 16mm and the root circle diameter of 16.9mm.

The properties of the threaded section 11 are summarised in Table 2 below and demonstrate that this threaded bar 10A would fail in the threaded region 11 at a breaking force of 116.1kn, i.e. well below the required minimum breaking force 202.8kN of the unthreaded region 3 of the bar 1, as indicated in the preceding Table 1.

In this case, 75% more threaded bars 10A would be required to provide the same design strength as unthreaded bars 1.

Table 2 A first preferred embodiment 110A and second embodiment 11 OB of the present invention are disclosed in Figs. 21-30, and Figs. 33-34 in which the elements which correspond to elements illustrated in Figs. 1-20 have a designation number increased by 100.

Figs. 21-22 disclose a threaded reinforcing bar 110A, made from a steel bar 101 of similar properties to the bar designated 1RB in Figs. 15-16 which has a distal end 102, an unthreaded region 103, an outside surface 104, a parallel threaded section 111 formed at an end 102 of the bar 101 by machining or by thread rolling the surface 104 for a determined length 113 and a nominal bar diameter 106. The threaded section 111 has a root circle 115 with a diameter 114 enclosing a root area 116 being the minimum cross-sectional area of the threaded bar 110A of Figs. 22b and 22b

After threading, the threaded end region 111 of the threaded bar 110A is introduced into a heating source, preferably an electric induction heater, represented diagrammatically by an induction coil 140 in Fig. 23. The heating source 140 is disposed about the threaded section 111 so as to ensure that the entire threaded length 113 is rapidly and uniformly heated to a temperature 141 above a transition temperature Ac (nominally 723°C) shown in Fig. 28.

Preferably the heating source 140 is an electric induction heater but could be any other type of heating source capable of rapidly heating the threaded end 111 to the desired temperature 141.

The heating source should preferably limit heating of the threaded bar 110A to the length 113 of the threaded section 111. Preferably the heating should be controlled to rapidly heat the threaded section 111 without significantly heating the adjacent unthreaded region 103 of the bar 101. The remainder of the bar 101 remains unheated so as to preserve the specified mechanical properties of the unthreaded section 103.

The heating source 140 may be adjusted to heat the threaded section 111 to the desired temperature 141 from the surface 104 of the threaded section 111 to a particular depth 136 and thereby create two zones, the first zone 137 being heated to the desired temperature 141 and a second zone 138 at a different temperature shown in Fig. 24. Alternatively, the threaded section 111 may be heated to the desired temperature to form only one zone 137 at the desired temperature 141 across the complete axial cross-section area of the threaded section 111.

After heating to the desired temperature 141 the heated zone 137 of the threaded bar 110A is rapidly cooled by quenching in water or other suitable quenching media, for a short period of time 143 to reduce the temperature sufficiently to harden a shell 130 of area 131 adjacent to the surface 104 so that the average strength of the threaded region 111 after cooling to ambient temperature will be equal to or higher than the unthreaded region 103 of the bar 101.

In Fig. 25 a spray device or devices (not shown) are disposed about the threaded section 111 and the adjacent unthreaded bar 103 in such a manner so as to direct water sprays 142 to uniformly quench the heated zone 137 of the threaded bar 110A. Effective quenching may be also be achieved by plunging the threaded section 110A and at least part of the adjacent unthreaded region 103 of the threaded bar 110A into an agitated bath of water or other quenching media for the required quenching period.

After quenching for the desired time 143, the threaded bar 110 is removed from the quenching device and allowed to air cool slowly over a long period 145 until it reaches ambient temperature 146. Fig. 27 represents the air cooling period where the arrows marked 145 represent the radiation of heat from the threaded bar 110A during the air cooling period 145.

Fig. 28 provides a time temperature graph of the surface temperature of the threaded section 111 when subjected to the preferred thermal process which minimises energy and cost by advantageously combining the hardening and tempering processes in one cycle.

It can be seen that after quenching the threaded bar 110A for the desired period of time 143, the surface temperature of the heated zone 137 rises from the quenched temperature 142 to a higher temperature 144 as a result of heat flow from the hot central core 131. This higher temperature 144 is sufficient to temper the steel of the shell 130, improving its strength, ductility and toughness.

In some circumstances a separate tempering process may be desired after quenching to ambient temperature 146, by reheating the threaded end 111 to a tempering temperature 144 below 600°C, followed by cooling to ambient temperature 146.

Figs. 29-30 disclose diagrammatic representations of the microstructures of the preferred, parallel threaded embodiment 110A of the current invention and a second embodiment 110B with a tapered thread 117, after thermal processing as described above and summarised in Fig. 28. It can be seen that the hardened shell 130 extends continuously from the unthreaded region 103 through the threaded region 111, 117 and that the soft ductile core 132 is completely enclosed and enveloped by the hardened shell 130 at the distal end 102 of the threaded bars 110A and 110B. The thickness of the hardened shell 130 in the threaded region 111,117 is controlled by adjusting temperatures 141, 142 and 144 and the quenching method and time 143, to retain ductility and ensure that the average strength of the threaded end 111,117 is greater than or equal to the strength of the unthreaded region 103 of the threaded bars 110A and 110B. A significant benefit of the above described method is that it may be adapted to threaded bars 110 for a wide variety of different thread geometries to meet the needs of individual applications and to maintain the strength of the threaded bar 110 at or above that of the parent bar 101.

This benefit is clear when threaded bars of the second embodiment 110B which have tapered threads 117 are considered. The prior art threaded bar 10B shown in Fig. 20 shows that taper threading 17 completely removes the hard shell 30 at the distal end 2 and reduces the average strength of the threaded section 17 to the low strength core steel 32. In contrast, reference to Fig.30 shows that the present invention restores the hard shell 130 through the threaded section 17 to ensure that the average strength of the threaded bar 11 OB is restored to the strength of the parent bar 101.

Figs. 31 shows a typical failure of a typical embodiment after a breaking force has been applied between the threaded section 111, of the end 102 of a threaded bar 110 and the non-threaded bar region 103. Failure 119 occurs in the unthreaded region 103 of the threaded bar 110.

Another advantage of such an embodiment is that the process hardens and toughens the resulting threads 111,117 which are supported by a strong hardened zone 130 below.

This increases the resistance to abrasion or damage. This is a significant benefit for threaded bars 110 used on construction sites where there is a high risk of handling damage. The hardened threads 111,117 improve the ease of connection because they are more resistant to galling with lower strength materials and hard enough to cut or crush materials fouling the threads.

Figs. 32-35 demonstrate further advantages of the preferred embodiments Fig. 32-33. show full-strength prior art threaded bars 10C with enlarged diameter threads 21 as shown in Fig. 12, joined together by a coupling device 50 of outside diameter 51.

Fig 34-35 shows full-strength threaded barsl 10 of the preferred embodiment of the present invention coupled with a coupling device 150 with an outside diameter 151.

Because the thread root diameter 24 of the prior art bars 10 is equal to or greater than the nominal bar diameter 6, from Fig. 32 it is obvious that the outside diameter 51 of the coupling device 50 will be larger than the outside diameter 151 of the coupling device 150 for the threaded bars 110 of the preferred embodiment with a thread root diameter 114 equal to or less than the diameter of the bar 101. The smaller coupling 150 saves material, energy and is less costly to manufacture than the larger prior art coupling 50. A further benefit is that the smaller coupling 150 of threaded bars 110 of the present invention enable coupled bars to be placed at closer axial centre distances than prior art threaded bars 10, thereby improving design efficiency and constructability.

Full-strength threaded bars 110 are required in many construction applications in addition to the effective coupling together of bars. A common application is for transferring load between a floor and a wall. Fig 36 shows a typical wall-floor joint 160 between a precast concrete wall 161 and a concrete slab floor 162 which is cast in place after the erection of the precast wall 161. The floor 162 is connected to the wall 161 by a threaded reinforcing bar 110 screwed into an internally threaded anchoring device 162 previously cast into the precast wall 161. The threaded reinforcing bar 110 is required to develop its full strength at the interface between the wall 161 and the floor 162 and maintain full ductility of the threaded bar when subjected to loads in excess of the design load.

Threaded bars 110 of the present invention provide ductile, full strength connections in wall-floor joints 160 and similar applications.

The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the arts, can be made thereto without departing from the scope of the present invention.

The term “comprising” (and its grammatical variations) as used herein is used in the inclusive sense of “including” or “having” and not in the exclusive sense of “consisting only of’.

Claims (29)

1. A method of forming a full strength thread on a steel bar of nominally circular transverse cross-section, said bar having a ductile interior and a thermally hardened exterior, said method comprising the steps of: (i) forming a thread having a plurality of turns at one end of said bar so as to remove some or all of said hardened exterior at said one end; and (ii) subsequently thermally heat treating said one end to thermally harden at least said thread but leave the interior of said bar inside said thread still substantially in its original ductile state, whereby the average strength of a transverse cross-section through said bar at a root of said thread is greater than or equal to the average strength of a transverse cross-section of the non-threaded remainder of said bar.

2. The method as claimed in claim 1 wherein said thermal treatment comprises heating the exterior of said one end in excess of a critical temperature.

3. The method as claimed in claim 2 wherein said critical temperature is approximately 723°C.

4. The method as claimed in claim 2 or 3 and including the subsequent step of quenching said heated exterior of said one end.

5. The method as claimed in claim 4 including the further step of tempering said one end after said quenching.

6. The method as claimed in claim 5 wherein said tempering comprises truncating said quenching and allowing the hot interior of said one end to heat the exterior of said one end.

7. The method as claimed in claim 5 or 6 wherein said tempering comprises heating said one end after said quenching.

8. The method as claimed in any one of claims 1-7 wherein said thread is tapered whereby said one end is frusto-conical.

9. The method as claimed in any one of claims 1-7 wherein said thread is parallel whereby said one end is substantially cylindrical.

10. The method as claimed in any one of claims 1-9 wherein said thread is cut by a metal cutting process.

11. The method as claimed in any one of claims 1-9 wherein said thread is rolled or forged or otherwise formed by a metal forming process.

12. The method as claimed in any one of claims 1-11 wherein said thread forming step results a transverse cross-sectional area through said bar at a root of any turn of said thread being less than or equal to a transverse cross-sectional area of the non-threaded remainder of said bar.

13. The method as claimed in any one of claims 1-11 wherein said thread forming step results in the transverse cross-sectional area through said bar at a root of any turn of said thread being greater than a transverse cross-sectional area of the non-threaded remainder of said bar.

14. The method as claimed in any one of claims 1-13 wherein said thermal treatment comprises in sequence: rapidly heating the end of the bar to a temperature in excess of 723°C, rapidly cooling the end of the bar, less rapidly heating the end of the bar to a temperature less than 723°C and thereafter slowly cooling the end of the bar.

15. A modified threaded steel bar of nominally circular transverse cross-section having a ductile interior and a thermally hardened exterior, said bar being modified by (i) forming a thread having a plurality of turns at one end of said bar so as to remove some or all of said hardened exterior at said one end, and (ii) subsequently thermally heat treating said one end to thermally harden by quenching and tempering at least said thread but leave the interior of said bar inside said thread still substantially in its original ductile state, whereby the average strength of a transverse cross-section through said bar at a root of said thread is greater than or equal to the average strength of a transverse cross-section of the non-threaded remainder of said bar.

16. A full strength threaded bar comprising a steel bar of nominally circular transverse cross-section, having a thread having a plurality of turns formed at one hardened end thereof to remove some or all of the exterior material, and having a ductile interior and a thermally hardened exterior, wherein said thread is thermally heat treated after forming to thermally harden said thread by quenching and tempering whereby the longitudinal tensile strength of said one end is greater than or equal to the longitudinal tensile strength of the non-threaded remainder of said bar.

17. The bar as claimed in claim 15 or 16 wherein said thermal treatment comprises heating the exterior of said one end in excess of a critical temperature.

18. The bar as claimed in claim 17 wherein said critical temperature is approximately 723°C.

19. The bar as claimed in claim 17 or 18 and including the subsequent step of quenching said heated exterior of said one end.

20. The bar as claimed in claim 19 including the further step of tempering said one end after said quenching.

21. The bar as claimed in claim 20 wherein said tempering comprises truncating said quenching and allowing the hot interior of said one end to heat the exterior of said one end.

22. The bar as claimed in claim 20 or 21 wherein said tempering comprises heating said one end after said quenching.

23. The bar as claimed in any one of claims 15-22 wherein said thread is tapered whereby said one end is frusto-conical.

24. The bar as claimed in any one of claims 15-22 wherein said thread is parallel whereby said one end is substantially cylindrical.

25. The bar as claimed in any one of claims 15-24 wherein said thread is cut by a metal cutting process.

26. The bar as claimed in any one of claims 15-24 wherein said thread is rolled or forged or otherwise formed by a metal forming process.

27. The bar as claimed in any one of claims 15-26 wherein said thread has a transverse cross-sectional area through said bar at a root of any turn of said thread being less than or equal to a transverse cross-sectional area of the non-threaded remainder of said bar.

28. The bar as claimed in any one of claims 15-26 wherein said thread has the transverse cross-sectional area through said bar at a root of any turn of said thread being greater than a transverse cross-sectional area of the non-threaded remainder of said bar.

29. The bar as claimed in any one of claims 15-28 wherein said thermal treatment comprises in sequence: rapidly heating the end of the bar to a temperature in excess of 723°C, rapidly cooling the end of the bar, less rapidly heating the end of the bar to a temperature less than 723°C and thereafter slowly cooling the end of the bar.