JPS6013046B2 - Heat treatment method for welded joints - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for heat treating a welded joint, particularly a welded joint made by welding low alloy steel and carbon steel.

When forming a joint by welding chromium-molybdenum (C-Mo) based low alloy steel and mild steel, heat treatment is performed to remove residual stress generated by welding and improve the material quality. The conventional heat treatment method is to heat the joint village in a furnace for 6
A method was used in which the temperature was gradually raised to around 00°C.

In this heat treatment method, the joint material is heated at a uniform temperature, so the temperature increases while the initial pillow state is maintained for the residual tensile stress generated during welding.

If the temperature is increased in such a state, the steel material will have a temperature of 300~
Since the material has a characteristic that intergranular cracking occurs in the temperature range of 55 to ○, the material becomes arms in this temperature range, and stress relaxation (Str
(hereinafter abbreviated as SR) had the drawback of causing cracks. The present invention aims to eliminate such problems and provide a heat treatment method for welded joints that can relieve stress and improve material quality without causing SR cracking. In a heat treatment method for a welded joint for removing tensile residual stress exceeding the yield stress occurring in a welded joint and improving the material quality, the welded part of the joint and the part other than the welded part are
The joint is heated while maintaining a temperature gradient such that the temperature of the welded part is higher than the temperature of the parts other than the welded part, and the thermal eaves and force generated by the heating cause a tensile stress that exceeds the yield stress during welding. An operation to cancel out the residual stress and make it a tensile or compressive residual stress below the yield stress, an operation to increase the temperature while maintaining the temperature gradient, and the highest temperature increased by this operation reaches the recrystallization temperature. After that, the method is characterized by comprising an operation of raising the temperature of the low temperature part while maintaining the maximum temperature to bring the entire temperature to the recrystallization temperature, and an operation of maintaining the recrystallization temperature for a predetermined period of time.

In other words, the present invention proposes that in order to prevent SR cracking, intergranular cracking can occur by reducing the tensile residual stress in the weld to less than the yield point, or by maintaining the weld in the opposite compressive residual stress state. This was done by focusing on the need to pass through a temperature range of 0 to 550 degrees Celsius.
This objective is achieved by creating a temperature gradient between the welded part and the parts other than the welded part to generate thermal stress that cancels out the tensile residual stress generated during welding.

For example, for welded gears, the main body of the gear is made of mild steel, which is flexible against impacts, etc., and the teeth are made of Cr-based material, which has high wear resistance.
It is manufactured by welding teeth to the gear body using Mo-based low alloy steel.
Generally 25k9/2 to 35k9, which exceeds the yield point of mild steel.
/ Welding tensile residual stress of about 2 ribs is generated.

Moreover, the material quality of the welded part is often considerably deteriorated due to rapid heating and cooling during welding. Therefore, it is necessary to perform SR and heat treatment to remove the welding residual stress of the welded gear and improve the material quality. However, if the welded gear is heated in the as-welded state, that is, with tensile residual stress exceeding the yield point in the welded part, the steel material will have tensile stress exceeding the yield point between 300o0 and 55000. This results in sudden SR cracking due to residual stress. For this reason, the temperature of the welding gear is 300oo~550℃.
SR cracking is prevented by preventing the presence of tensile residual stress exceeding the yield point when passing through the temperature range of 0.00. For example, the temperature of the welded part is 100 oo higher than the temperature of the other part.
Then, when the linear expansion coefficient of steel is 1.0xlo-5/℃, the thermal strainer becomes 1000x10-6 peaks, and at this time, the welded part has the m formula y=E......(
The compressive stress determined from 11 acts. That is, E
(Young's modulus) = 20,000kg/f/Yanagi2, y
=20,000×1,000×10-6=20k9f/
Willow 2, the tensile residual stress of the welded part is 20k9f/side 2
25-35k generated in the welded area can be reduced.
The tensile residual stress of 9f/fence 2 is as small as 5 to 15 kgf/fence 2. Since the yield stress of the welded part is 15k9f/fence 2 or more, SR cracking can be prevented by increasing the temperature while maintaining the temperature difference. Examples will be described below.

FIGS. 1 and 2 show a heat treatment apparatus for carrying out an embodiment, in which teeth 1 made of Cr-Mo low alloy steel
Fig. 2 shows the heat treatment state of a welded gear formed by welding the welded gear to a gear body 2 made of mild steel at a welding part 3, and Fig. 2 shows a cross section A--A of Fig. 1.

The welding gear is placed on a fixed table 6 of a heat treatment furnace 5, and heaters 7 made of heating wires are provided on both sides of the welded part 3 of the welding gear in close proximity to each other. A pipe 8 is provided for supplying air. Further, the heat treatment furnace 5 is provided with an in-furnace thermocouple 9 for measuring the temperature inside the furnace, a welding part thermocouple 10 for measuring the temperature of the welded part, and a low-temperature part thermocouple 11 for measuring the temperature of the low-temperature part. A thermocouple thermometer 12 makes it possible to measure the temperature of each part. In order to heat-treat a welded gear using such a heat treatment furnace 5, the welded part 3 of the welded gear is heated with a heater 7, and the gear body 2 that is away from the welded part 3 is heated with cooling air through a pipe 8. The temperature of the gear body 2 is always about 100 oo lower than the temperature of the welded part 3 by cooling the gear body 2.

These operations are performed by measuring the detection results of the welding part thermocouple 10 and low temperature thermocouple 11 with the thermocouple thermometer 12, and measuring the power of the heater 7 and the amount of cooling air supplied to the pipe 8. It is done in a controlled manner. In this way, the tensile residual stress generated in the weld 3 due to the thermal stress generated by creating a temperature gradient between the weld 3 and the gear body 2 can be reduced by approximately 20k9/Yanagi2, far below the yield point. Can be with small tensile residual stress. Figures 3 and 4 schematically show the distribution of residual stress in the welded gear under this excess condition, and the same parts as in Figures 1 and 2 are given the same symbols, and Figure 3 Figure 4 shows the stress distribution when a temperature gradient is applied between the welded part and the part other than the welded part to make the welded part high temperature. indicates tensile stress and C indicates compressive stress.

Next, while maintaining this temperature gradient, the welding gear is gradually raised to 625° C. (recrystallization temperature).

This operation is carried out by detecting the temperature inside the furnace with the furnace heat crab pair 9, and when the temperature of the welding part 3 rises to 625°C, the heater 7 is turned on to prevent the temperature of the welding part 3 from rising any further. The temperature of the welding part 3 is maintained at 625 oo by control. At this point, the temperature of the gear body 2 is 52yo, but next, the supply of cooling air flowing through the pipe 8 is stopped or reduced, and the temperature of the gear body 2 is gradually increased to lower the temperature of the entire welded gear. At a uniform temperature of 625℃, keep it at that temperature for 3 to 4 hours.
Hold time. Due to uniform heating at this recrystallization temperature,
It is possible to recrystallize a welded part where the material has deteriorated due to rapid heating and cooling during welding, adjust the crystal structure, and completely remove residual stress from welding. In such a heat treatment method, compressive stress is generated in the weld by a temperature gradient applied between the weld and the parts other than the weld, and this reduces the residual tensile stress generated during welding to below the yield point. Alternatively, the residual tensile stress can be converted into compressive stress and passed through a temperature range of 300 pm to 550 qo where SR cracking occurs in that state, making it possible to prevent SR cracking. That is, by using the method of this embodiment, it is possible to remove residual stress in the weld and improve the material quality without generating SR cracking, and the reliability of the strength of the product is greatly improved.

In addition, although the example shows an example of Cr-Mo based low alloy steel, SR cracking is applied to low manganese steel, silicon-manganese steel, manganese-chromium steel, manganese, molybdenum steel, manganese-chromium-molybdenum steel, and nickel.・This heat treatment method can be applied in the same way, as this generally occurs in low alloy steels such as chromium-molybdenum steels.

However, since SR cracking is particularly noticeable in Cr-Mo steel, the effect is particularly noticeable. In addition, although this example shows an example of a welded gear, it is widely applicable to welded joints made by welding low alloy steel and carbon steel.
A similar effect can be obtained. As described above, the heat treatment method for welded joints of the present invention enables stress relief and material quality improvement without causing SR cracking, and has great industrial effects.

[Brief explanation of drawings]

Fig. 1 is a sectional view of a heat treatment apparatus for carrying out an embodiment of the heat treatment method for welded joints of the present invention, Fig. 2 is a sectional view taken along A-A in Fig. 1, and Fig. 3 is a sectional view of a welding gear during welding. Fig. 4 is a stress distribution diagram of a welded gear in one step of the heat treatment method for a welded joint according to the present invention. 1... Teeth, 2... Gear body, 3... Welded part, 4... Shaft hole, 5.
... Heat treatment furnace, 7 ... Heater, 8 ... (For supplying cooling air) pipe. Multi-figure multi-figure 2-figure multi-figure 3-figure multi-figure 4

Claims (1)

[Scope of Claims] 1. A heat treatment method for a welded joint for removing tensile residual stress exceeding the yield stress occurring in a joint made by welding low alloy steel and carbon steel and improving the material quality, the method comprising: and a part other than the welded part are heated while maintaining a temperature gradient such that the temperature of the welded part of the joint is higher than the temperature of the part other than the welded part, and the thermal stress generated by the heating An operation to cancel the tensile residual stress above the yield stress during welding to a tensile residual stress or compressive residual stress below the yield stress, an operation to increase the temperature while maintaining the temperature gradient, and an operation to increase the temperature increased by the operation. After the maximum temperature reaches the recrystallization temperature, the method includes an operation of increasing the temperature of the low temperature part while maintaining the maximum temperature to bring the entire body to the recrystallization temperature, and an operation of maintaining the recrystallization temperature for a predetermined distance. Characteristic heat treatment method for welded joints. 2. The method of heat treating a welded joint according to claim 1, wherein the low alloy steel is a chromium-molybdenum based low alloy steel, and the carbon steel is a mild steel.