JPS6320908B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is mainly a treatment applied to processed parts of standard nitriding steel, structural carbon steel, general structural alloy steel, and other steel materials, but it also improves the durability of finished products of general tool steel such as taps and various molds. The present invention relates to a nitrided layer-stabilized steam coating treatment method and apparatus that further improves durability, wear resistance, fatigue resistance, etc. by adding pressurized nitriding and superheated steam treatment as additional heat treatment for further improvement. Regarding conventional nitriding and pressure nitriding treatments for steel materials, see Japanese Patent Publication No. 28-6262, Japanese Patent Application Laid-open No. 49-91047,
JP-A-52-145343 and JP-A-52-145343 are known, and steam treatment for forming an iron oxide film on the outermost surface of steel after nitriding is disclosed in JP-A-51-2635. Publication No. and Special Publication No. 53-371
It is proposed in the Publication No. However, Japanese Patent Application Laid-Open No. 51-2635 states that ``The temperature inside the gas soft-nitriding furnace 11 is 520℃ during the gas soft-nitriding process.
After raising the temperature to a gas soft-nitriding temperature of ~600℃ and dropping the granular urea into the furnace, the gas soft-nitriding treatment is performed using the decomposed gas generated.The treated material is then left in the furnace and the temperature inside the furnace is lowered. The temperature is lowered to 200℃ to 300℃, which is necessary for steam treatment, and then superheated steam generated in the steam generation tube is blown into the furnace. A part of the water vapor decomposes into 2H <sub>2</sub> O → 2H <sub>2</sub> + O <sub>2</sub> on the surface of the treated object, and the desired number is formed on the compound layer on the surface of the nitrocarburized object by the nascent O <sub>2</sub> . Micron
A Fe <sub>3</sub> O <sub>4</sub> film is formed. ", which is seemingly similar to the present invention, but the above 200℃~300℃
In water vapor, an extremely complex oxide film with large particles is formed and adheres to the stable triiron tetroxide (Fe <sub>3</sub> O <sub>4 )</sub> .
A so-called black rust film is not formed, and red rust does not contribute in any way to the durability of cutting tools, etc., and has no effect on friction during cutting, etc. In addition, Japanese Patent Publication No. 53-371 states, ``After the steel material is heated in a mixture of nitriding gas and carburizing gas at 500°C to 600°C, it is held in air for 60 to 120 seconds to produce triiron tetroxide. "Corrosion-resistant surface treatment method for steel materials that produces a film"
However, oxidation does not occur in nitriding gas or carburizing gas, and the temperature for carburizing with carburizing gas is at least 800℃ or higher, and the temperature for nitriding with nitriding gas is 500℃ as mentioned above. It is independent of carburizing gases at ~600°C. Furthermore, as mentioned above, the temperature range of 500℃ to 600℃ is α
The condition is such that decarburization and carburization are not promoted in the iron region, and the oxidation retention time of the treated material is extremely short;
The cooling rate differs depending on the shape and size, and the oxidation effect is completely different.Furthermore, the conditions required for oxidation in the atmosphere vary depending on the humidity level depending on the season, resulting in different results, making the process very unstable and complicated. A stable triiron tetroxide film is not produced as an iron oxide film. The purpose of the present invention is to solve the above-mentioned drawbacks by forming a triiron tetroxide coating of fine particles on steel materials after pressure nitriding treatment by continuous superheated steam treatment in the same furnace. The purpose is to reduce the coefficient of friction by creating both hardness due to nitrides and a fine particle triiron tetroxide coating (Fe ₃ O ₄ ), and to improve the relationship with the hardness of the steel material. . Another object of the present invention is to facilitate the formation of a triiron tetroxide film on fine particles in a nitriding furnace. The feature of the present invention is that the steel material is subjected to pressure nitriding treatment using a nitriding gas produced by decomposing ammonia at a furnace temperature of 450°C to 600°C, and after the nitriding treatment, the ammonia gas in the furnace is removed from the furnace. , continuously replaced with superheated steam and held in superheated steam at 450°C to 600°C for 30 to 60 minutes to form a fine particle triiron tetroxide coating (Fe ₃ O ₄ ) on the surface of the steel material. It is. Hereinafter, the present invention will be described in detail with reference to examples. First, in pressurized nitriding, ammonia gas is led from an ammonia cylinder to an intermediate tank, where it is heated and pressurized, and then supplied to a processing furnace to raise the temperature of the steel to be treated from 450℃ to 600℃ as shown in Figure 1. At the same time, the gas pressure was increased from 7Kg/ ^cm2 to 100Kg/ ^cm2 , and the nitriding treatment was performed for a treatment time T1 of 1 to 10 hours.
Subsequently, superheated steam treatment is performed to form a triiron tetroxide film of fine particles. In the superheated steam treatment, after the pressurized nitriding treatment is completed, nitrogen gas _N2 is continuously pumped in to eliminate nitriding gas caused by the decomposition of ammonia in the treatment furnace.
After the ammonia gas has escaped, superheated steam is continuously ejected from the pores of the piping provided in the processing furnace until time T ₂ in FIG. 1 to replace the nitrogen gas N ₂ with superheated steam. Temperature after complete replacement to 450℃
The temperature is maintained at 600°C and the time from _T2 to _T3 is maintained for 30 to 60 minutes, and after the superheated steam treatment, the steel material to be treated is left to cool or cooled in oil. The steel materials used in the above treatment include nitriding standard steel, structural carbon steel, general structural alloy steel, mold alloy steel, high-speed tool steel, and finished products such as tubs and precision parts for machinery made of these steels. Subject to processing. As a result of the above-mentioned pressurized nitriding treatment + superheated steam treatment, fine particles of triiron tetroxide are formed on the surface of the steel to be treated, as shown in the cross-sectional photograph of the steel material after superheated steam treatment in Figure 2.
A film of Fe ₃ O ₄ can be produced. Figure 3 is an X-ray diffraction diagram of the surface layer of a steel material that has been subjected to pressure nitriding and steam treatment in succession, showing that various nitrides and triiron tetroxide are produced in a mixed state on the same surface layer. is recognized. Furthermore, Figure 4 shows the relationship between long-distance wear and wear amount using the sliding wear method using a combination of a rotating test piece and a stationary test piece.The rotating test piece was made of S55C structural carbon steel, and Afterwards, it was subjected to pressure nitriding treatment to obtain a hardness of Hv570, and SKH9 high-speed tool steel was used as the fixed specimen. As additional heat treatments, we added four types of composite heat treatments with different conditions, such as pressurized nitriding and superheated steam treatment, and combined these four types with the former as fixed test pieces to conduct sliding wear experiments with the addition of lubricating oil. There is. As shown in FIG. 4, the contents of the fixed test piece were individually conducted for each heat-treated steel, and the maximum allowable contact pressure for each heat-treated steel was determined. Sample is 25Kg/cm ² , 30Kg/cm ² , 50Kg/
cm ² , was found to be 60Kg/cm ² . The above lubricating oil is paraffin-based #60 spindle oil (oil temperature 20-22℃), the surface roughness of the steel specimen;
Rmax0.3~0.4μm, pressure nitriding treatment: 520℃×3h,
Superheated steam treatment at a constant pressure of 7 atm: After preheating to 350°C to 400°C, the temperature was further raised and steam treatment was performed at 520°C for 30 minutes. Now, under each different load, only the friction velocity is
Wear is being tested at a constant speed of 3.4 m/s. According to this result, as is clear from Figure 4, sample (iv) (steel in which SKH9 was quenched, tempered, pressure nitrided, and then continuously subjected to superheated steam treatment) had a contact pressure of 60 kg. /cm ² , it was confirmed that the amount of wear was reduced the most, as shown by the black circle, even though the load was maximum. The above pressure nitriding treatment was performed at 520°C, 7 atm, and for 3 hours.
Superheated steam treatment was performed at 520°C for 3 minutes. As shown by the black circle curve, the results show that the wear amount of the fixed test piece that was sequentially subjected to the pressure nitriding treatment and superheated steam treatment of the present invention was significantly improved compared to the results of other treated test pieces. . Next, an example of a screw cutting experiment in which the composite heat treatment conditions of the present invention were applied to a cutting tool was shown. Superheated steam treatment is carried out continuously, the tap diameter is 10 mmφ, and the tap is previously subjected to industrial standard quenching and tempering treatment in a salt bath according to the following conditions, and as an additional heat treatment to the tap, the pressurized nitriding of the present invention is further applied. A thread cutting experiment was conducted on the steel to be cut by sequentially performing treatment and superheated steam treatment in accordance with JIS regulations. Tap material: (SKH9 used in the above wear experiment) 0.85% C, 0.31% Si, 0.32% Mn, 0.02% P,
0.005%S, 4.1%Cr, 6.2%W, 5.2%Mo, 1.92%
Industrial standard heat treatment process in V tap salt bath, 850℃ → 30 minutes preheating → 1200℃ → 10 minutes quenching heating →
Isothermal quenching in a salt bath at 520℃ (this quenching method is used to reduce cracking or deformation) → After cooling at room temperature → Tempering treatment at 550℃ twice for 1.5 hours each. The resulting hardness is H _R C66-67. The cutting experiment was conducted by drilling a pilot hole in the steel to be cut and using the tap described above. As a result, the durability of steels that are easily work-hardened, such as 18-8 austenitic stainless steel and case-hardened steels such as SCM21, is about twice as strong, while the durability of steels that do not have to worry about seizure, such as S45C structural carbon steel, is about 2 to 2 times more durable. Approximately four times the durability was obtained. The main application of the present invention is wear resistance, and as can be seen from the above cutting experiment examples, the general taps that have been hardened and tempered are usually subjected to heat treatment and finishing in order to further improve their cutting durability. The objective is to add additional heat treatment to the finished product after processing, thereby increasing the useful life of the tap. In other words, it means adding an additional heat treatment to the finished tool, and the content of the +α treatment is, for example,
After pressure nitriding at 520℃ for 1 to 3 hours, continuous superheated steam treatment at the same temperature of 520℃ for 30 to 60 minutes gives the top surface layer of the tap tool a solid Fe _o N _n This process produces a mixture of nitride and fine particles of triiron tetroxide (Fe ₃ O _{4 )} . In addition, in the case of jigs and tools made of various tool steels such as cutting tools and alloy steel for molds, precision machine parts, and parts that require wear resistance and fatigue resistance, the above-mentioned finished products are subjected to additional processing. However, for parts whose steel type is suitable for the nitriding effect, direct pressure nitriding treatment + superheated steam treatment may be applied to improve wear resistance, fatigue resistance, etc. . Figure 6 is an example of an S-N diagram, with a white circle at 520°C.
It is known that nitrided steel has the best fatigue resistance. As mentioned above, in the present invention, the steel material to be treated by the pressure nitriding method in the first half is treated with superheated steam at the same temperature with a slight time delay to form four fine particles on the surface of the nitrided layer structure. When a continuous heat treatment is applied to precipitate iron trioxide, the hardness of the nitrided layer on the surface has a good relationship between the hardness gradient and the hardness of the substrate (it becomes a gentle curve), and when the cutting tool is subjected to the above treatment, Chips on the cutting edge gradually decrease. This is also a feature of Fe ₃ O ₄ shown in wear resistance experiments; the fine particle triiron tetroxide film acts like a solid lubricant, lowering the friction coefficient and reducing the friction temperature, reducing the amount of wear. It is to decrease. The point is that the temperature of the cutting edge does not rise.
Durability is significantly improved. This effect has a similar effect on various molds, etc., and particularly improves the mold releasability for molds with complicated cavity shapes, such as molds for synthetic resin molding. Furthermore, if the steel material to be treated which has been subjected to the pressure nitriding treatment is taken out in the middle of the process before superheated steam treatment is carried out and its condition is examined, it will be found that a small hole is made in the steel material and placed in a position opposite to the flow direction of the nitriding gas. However, according to the cross section after treatment, a uniform nitrided layer is formed on the inner pore wall surface of the pore as shown in FIG. In short, even if the surface shape is significantly uneven, this nitriding method can provide a uniform nitrided layer along the surface of the steel to be treated. The content of Figure 6 on the previous page is that when a steel material JIS, SCM3 is subjected to pressure nitriding at a pressure of 7 atm, a gas decomposition amount of 30%, and a temperature of 510℃ to 550℃ and repeated stress experiments are performed, the results shown in Figure 6 are obtained. The treatment temperature is around 510°C to 520°C (black and white circles), which gives good fatigue resistance. Furthermore, as shown in Table 1, compared to general nitriding using carrier gas (RX gas) and ammonia (NH ₃ ), pressurized nitriding increases surface hardness, nitriding depth, compressive residual stress, and fatigue limit. If the pressure nitriding temperature is higher than 600°C, the hardness will drop rapidly, and if it is lower than 450°C, the treatment time will be long, which is not industrially practical, and the hardness will not increase in proportion to the treatment time. Regarding the processing stress, since ammonia gas is used, a stress of 100 kg/cm ² or more is difficult to handle industrially. The advantage of pressurized nitriding treatment over general nitriding shown in Table 1 above is that the nitriding resolution is somewhat suppressed by pressurization, but under pressure, the reaction of NH ₃ 〓3H + N゜ is repeated from side to side, so NH ₃ consumption is extremely high. The amount is small and resource saving. In addition, active nitrogen (original) due to pressure

[Table] Increases the diffusivity of nitrogen particles (N゜). In other words, a layer similar to high-temperature diffusion is uniformly generated even on the uneven portions without reducing the nitriding hardness. After the above-mentioned pressure nitriding treatment, the steel material to be treated is
53-371, exposing it to air,
When the temperature of superheated steam cannot be maintained at a constant temperature within the range of 450°C to 550°C, as in JP-A No. 51-2635, a film of large particles of triiron tetroxide or ferric oxide is formed. The above effect cannot be obtained. An example of a processing furnace for the above-mentioned combined heat treatment of pressurized nitriding and superheated steam is explained with reference to FIG. Heated air is circulated, and a processing tank 13 is disposed at the center of the processing furnace 10. The processing tank is a completely sealed type made of a heat-resistant and corrosion-resistant alloy, and the processing tank is uniformly heated.
The outer periphery of the yoke 14 provided above the outer periphery of the processing tank 13 is placed on a sand container 15 provided on the upper surface of the processing furnace 10 and subjected to sand packing. At the bottom of the processing tank 13 is a nitriding gas supply pipe 1.
6 is connected, and a superheated steam supply pipe 17 is spirally wound around the outer periphery of the processing tank 13.
The lower end of is connected to the nitriding gas supply pipe 16. A pipe 18 for exhausting the nitriding gas is connected to the upper end of the processing tank 13. Processing furnace 10
A thermocouple 19 is inserted into the processing furnace at a proper position. The object to be processed is the upper lid 20 of the processing tank 13.
Open and insert. The pipe 16 is connected to a pipe in the processing tank 13 that has a superheated steam jetting hole. The pipe 17 having pores may be directly connected separately from the pipe 16. The temperature control of the processing furnace for combined heat treatment and the replacement of ammonia gas and superheated steam may be performed manually, or may be automated using a combination of an automatic temperature controller, a timer, and a program computer. Since the present invention is configured as described above, when pressure nitriding treatment and superheated steam treatment are performed in succession, fine particles of triiron tetroxide are added to the uniform nitrided layer on the outermost surface formed by pressure nitriding treatment. A film is generated, and this film acts like a solid lubricant, reducing the coefficient of dynamic friction and maintaining wear resistance.The pressure nitriding treatment also reduces compressive residual stress that stably exists and provides extremely high resistance. A nitrided layer that has fatigue resistance and quasi-austenitic properties that provide excellent wear resistance even when the hardness is relatively low, as well as improved durability, wear resistance, fatigue resistance, etc. A stabilized steam coating treatment method and apparatus can be provided.

[Brief explanation of drawings]

The drawings show an embodiment of the nitrided layer-stabilized steam coating treatment method and apparatus of the present invention, and FIG. 1 is an explanatory diagram of the nitrided layer-stabilized steam coating treatment process, and FIG. 2 is a diagram showing the process after pressure nitriding treatment + superheated steam treatment. Cross-sectional photo of steel material, 3rd
The figure shows an X-ray diffraction diagram of the surface layer of the steel material subjected to pressure nitriding treatment + superheated steam treatment in succession. Figure 4 shows quenching/tempering treatment, pressure nitriding treatment, pressure nitriding treatment + superheated steam treatment. Graph of the relationship between wear amount in long-distance wear after continuous pressure nitriding treatment + superheated steam treatment, Figure 5 is a cross-sectional photograph of the steel to be treated after pressure nitriding treatment, and Figure 6 is pressure treatment. The S-N diagram of the repeated stress of steel, FIG. 7 is an explanatory diagram of the processing furnace. 10...processing furnace, 13...processing tank.

Claims (1)

[Claims] 1. Steel material is exposed to 7 kg/cm 2 to 7 kg/cm ² of nitriding gas due to decomposition of ammonia in a furnace temperature of 450° C. to 600° C.
Nitriding treatment is performed by applying a pressure of 100Kg/ ^cm2 ,
After the nitriding treatment, the ammonia gas in the furnace is removed and continuously replaced with superheated steam at a temperature of 450℃ to 600℃.
A nitride layer-stabilized steam film treatment method characterized by holding the steel in superheated steam for 30 to 60 minutes to form a triiron tetroxide film of fine particles on the surface of the steel material. 2. The processing furnace for complex heat treatment is capable of smooth operation in accordance with its processing characteristics, and is characterized by being constructed with a completely sealed processing tank made of a heat-resistant and corrosion-resistant alloy. Device.