GB2160227A

GB2160227A - Heat treatment process

Info

Publication number: GB2160227A
Application number: GB08411495A
Authority: GB
Inventors: John Durham Hawkes
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-05-04
Filing date: 1984-05-04
Publication date: 1985-12-18
Also published as: GB2160227B; GB8411495D0

Abstract

A heat treatment process for transformation hardening provides a line heat-input pattern which is scanned along the surface of the material being treated, so that the area of the surface which is continuously heated is substantially elongate in the scanning direction. The scanning is carried out at a speed which allows sufficient heat input and heat-dwell time for the required solid-state phase- transformation to take place. The method is of particular application of the hardening of an edge region of a strip S of thin-section material. The elongate heat-input pattern P is scanned along the edge of the strip S, for example employing an induction or laser beam heating system, which produces a substantially uniform heat input over the elongate heated area. <IMAGE>

Description

SPECIFICATION Heat treatment process The invention relates to heat treatment processes and is particularly, but by no means exclusively, concerned with so-called "transformation" hardening of a longitudinal edge of thin strip material employing induction heating or a laser beam to heat the material.

Transformation hardening of steel is a welldeveloped process. The material is heated for a sufficient time to allow solution of carbide with formation of austenite, the heated steel then being quenched to transform the austenite into the hard substance known as martensite. With carbon steels the heating is typically to a temperature of 800-900"C (depending on the carbon content) and cooling during the quench period must exceed what is known as the "critical cooling rate".

In such a process induction or eddy-current heating is commonly employed with the surface area to be heated, such as the edge of a blade, scanned with an induction heating head which rapidly heats an area of the surface in proximity to the inductor to the requisite temperature, this area being kept small in the scanning direction and the head normally being followed by quench jets. However, problems have arisen in the application of such a process to the hardening of martensitic stainless steel, that is to alloy steels which principally contain 11-13 wt % Cr and 0.1-1.2 wt % C where the chromium content provides the oxidation and corrosion resistance required, for example, for cutlery and other domestic articles.The surface must be heated to a higher temperature, typically of about 1 050 C for steel of 12 wt % Cr and 0.2 wt % C for example, whereas the melting temperature is slighly reduced so that the "operating window" for complete austenization without surface melting is somewhat smaller. The reaction time at the heated temperature is significantly increased, due to slower kinetics for the dissolution of the carbide phases in the presence of chromium compared to that for normal carbon steels, whereas due to the sluggish growth of chromium related carbides on cooling (as well as their dissolution on heating) the critical cooling rate that must be exceeded to produce a martensitic hardened case is relatively low.

Due to the foregoing changed characteristics, an induction heating process as nor mally used for carbon steels cannot satisfactorily be employed with martensitic stainless steel when a high scanning rate is required, and the object of the invention is to provide such a selective heat treatment process which provides good results with such material. A low scanning rate would not only be uneconomic but would produce a deeper case and a larger "soak" time, and thus in the case of a blade edge the latter would be badly distorted due to the release of residual stresses induced during the forming process.

According to the invention a heat treatment process for transformation hardening provides a line heat-input pattern which is scanned along the surface of the material being treated, so that the area of the surface which is continuously heated is substantially elongate in the scanning direction.

The use of the line heat-input pattern of the invention enables satisfactory transformation hardening of martensitic stainless steel alloys.

In particular it allows sufficient heat input and heat-dwell time for solid-state phase-transformation to take place, while maintaining a low input-power density and therefore a narrow affected "case" without exceeding that inputpower density which would damage the surface or cause distortion, and also without reducing the scanning rate. The "heat-dwell time" is the time for which an individual point on the surface is heated, which is the time for the elongate heating pattern to pass over the point and this is equal to the length of the pattern divided by the scanning rate.

The invention is very conveniently employed for the case hardening of the edge of a stainless steel scissor blade, for example, with the elongate heating pattern being scanned along the cutting edge of the blade formed from stainless steel strip. Although induction heating is suitably employed to produce the elongate heating pattern by eddy-current heating of the material, the invention can also be employed utilizing a laser beam for production of the heating pattern. To this end the laser beam from a carbon dioxide or solid state laser may be directed through suitable optics on to the surface of the metal to be hardened, producing an elongate line pattern. The surface of the metal may be coated with a suitable substance to aid the beam absorption and cause efficient heating. Black paint, colloidal graphite or a phosphate coating are suitable substances for this purpose.Suitable optical configurations which may be used are a cylindrical focussing lens, a rotating polygon mirror device and an oscillating mirror.

Another suitable application for the invention is for the case hardening of a strip along the surface of a steel body, for example to harden the bearing race track of a linear roller bearing. With laser beam heating employing a rotating polygon mirror or oscillating mirror a large depth of focus is available and thus not only flat surfaces and edges but also cylinders can be heat treated, and the elongate heating pattern can follow a spiral around the cylindri cal component.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which: Figure 1 shows the invention applied to the hardening of an edge region of a thin strip component; Figure 2 shows the invention applied to the hardening of the surface of a cylindrical component; and Figure 3 illustrates a modification of the process as shown in Fig. 2.

Fig. 1 shows the invention applied to the hardening of an edge region of a strip S of thin-section material. An elongate heat-input pattern P is scanned along the edge E of the strip S which is to be hardened, employing an induction heating system which produces a substantially uniform heat input over the elongate heated area. Scanning is carried out at a speed which allows sufficient heat input and heat-dwell time for the required solid-state phase-transformation to take place.

The scanning may be followed by quenching with quench jets in the normal manner, to transform the austenite produced during the heating time to a surface layer of martensite across the full width of the edge E. If laser hardening is employed as an alternative to the induction heating no external quench is required as the laser energy is absorbed at the optical surface and moves into the metal purely by conduction. This results in much higher thermal gradients near the metal surface and hence the mass of cold metal below acts as an efficient quenching agent.

The length L of the elongate heated area is many times greater thap the width W of the edge E, typically being about 1 5 times greater. The thin strip S is of martensitic stainless steel, for example 420S45 alloy which contains 1 2 wt % Cr and 0.2 wt % C and is very suitable for cutlery and domestic articles generally, for example for scissor blades.

In particular the described process fulfils the requirements of controlled heating to a higher temperature but with increased dwell time as compared with a normal carbon steel, but without a corresponding decrease in the scanning rate. Furthermore, with this material the quench rate is not as critical as is the case with carbon steels and thus the form of the heating cycle is more easily changed, as in the present invention, without hindering quench requirement considerations.

Figs. 1 and 2 show the invention applied to the surface hardening of a cylindrical steel component C, conveniently employing heating by means of a laser beam. In the process of Fig. 2 a laser beam of adequate depth of focus produces an elongate heating pattern P which spans the full diameter width of the cylinder C and which is disposed in a plane inclined at an angle 8 to the longitudinal axis of the cylinder C. The cylinder is traversed longitudinally as shown by the arrow T and simultaneously rotated as shown by the arrow R, so that the elongate pattern follows a spiral around the cylindrical surface S of the component.

As a result of the combined longitudinal traverse and rotation of the component C a point on the surface S will follow a locus L which, as shown in Fig. 2, is not quite straight in horizontal projection. However, as can be seen from that figure the deviation of the locus L from-the straight is contained within the width of the heating pattern P.

Because of the geometrical constraint it is generally preferable to shorten the heating pattern as shown in Fig. 2 so that in horizontal projection it spans only a central region r of the cylinder C. In that figure alternative heating patterns P1 and P2 are shown of different lengths, the inclination angle being varied from 8, to 82 SO that the pattern still spans the central region r of the component.

In addition to vartiation of this angle, the speeds of traverse T and rotation R can be varied to suit the pattern and beam energy for satisfactory hardening of the particular material being treated.

Claims

1. A heat treatment process for transformation hardening which provides a line heatinput pattern which is scanned along the surface of the material being treated, so that the area of the surface which is continuously heated is substantially elongate in the scanning direction.

2. A heat treatment process substantially as herein particularly described with reference to the accompanying drawings.