JPS6013047B2 - 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 koji.

When forming a joint by welding chromium-molybdenum (Cr-Mo) low alloy steel and mild steel, heat treatment is performed to remove residual stress generated by welding and improve the material quality. The heat treatment method is to heat the joint material 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 maintaining the initial state of the residual stress and tensile stress generated during welding.

If the temperature is increased under these conditions, the steel material will reach a temperature of 300°C.
Because the material has a characteristic that intergranular cracking occurs in the temperature range of ~550°C, the material becomes arms in this temperature range, and stress relaxation (Stress Rela Matsu) tfon) (hereinafter abbreviated as SR) There was a 0 point that caused cracking. 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 occurring in a welded joint and improving the material quality, only the parts of the joint other than the welded part are heated, and the welded joint is heated between the welded part and the welded joint. The process of applying a temperature difference that causes the tensile residual stress occurring in the joint to yield on the tensile side, and then cooling it to room temperature to change the tensile residual stress generated during welding into compressive residual stress by reversing the stress, and how to perform this operation. This method is characterized by comprising an operation of gradually raising the temperature of the entire welded joint to a recrystallization temperature, and an operation of holding the welded joint at the recrystallization temperature for a predetermined period of time.

In order to prevent SR cracking, the present invention is to change the tensile residual stress in the weld zone to the opposite compressive residual stress, which will cause intergranular cracking.30
This was done by focusing on the fact that it is necessary to pass through a temperature range of 0oo to 550qo.

That is, if a temperature difference is applied that causes the residual stress in the weld to yield on the tensile side, the stress is reversed during the subsequent cooling process to room temperature, and the weld can be placed in a compressive residual stress state.

Therefore, even if the welded joint is heated uniformly thereafter, SR cracking will not occur. For example, the temperature of the area other than the welded area is 150° higher than that of the welded area.
If qo is increased, the thermal expansion will be 1500 x 10-6 when the linear expansion number of Qi is 1.0 x 10-5/℃, and at this time, the welded part will have a
A tensile stress calculated from """m acts. That is, E (Young's modulus); 20,000k9/f/side 2, y = 20,000 x 1500 x 10-6 = 30k9f
/Kaede, and the welded part is completely in a tensile yield state at 30k9f/side 2. Thereafter, by cooling, stress reversal occurs and the welded part becomes in a compressive residual stress state at room temperature, so even if the whole is uniformly heated to the recrystallization temperature afterwards, no SR cracking will occur. In addition, uniform heating at the recrystallization temperature can recrystallize the material that has deteriorated due to rapid heating and cooling during welding, adjusting the crystal structure and completely eliminating residual stress from welding. .

For example, welded gears use mild steel for the gear body, which is flexible against impacts, etc., and the teeth are made of Cr-M, which has high wear resistance.
It is manufactured by welding the teeth to the gear body using o-series low alloy steel, but the weld between the gear body and the teeth is generally 25k9/fence 2 to 35k, which exceeds the yield point of European steel.
9/Yanagi Welding tensile residual stress of about 2 has occurred.

Moreover, the material quality of the welded part is often considerably deteriorated due to the rapid heating during welding. Therefore, it is necessary to perform SR and heat treatment to remove the residual stress of the gear and improve the material quality. However, if the gear is heated as welded, that is, with tensile residual stress exceeding the drop point in the weld, the steel material will heat up to 300°C.
SR cracking suddenly occurs due to tensile residual stress exceeding the deterioration point between 550°C and 550°C. Therefore, the present invention prevents SR cracking by preventing tensile residual stress exceeding the yield point when the gear temperature passes through a temperature range of 300qo to 550°C. I will explain about it. Fig. 1 and Fig. 2 show the implementation status of one embodiment, which is a welded gear in which teeth 1 made of Cr-Mo low alloy steel are welded to a gear body 2 made of European steel at a welding part 3. Shows the heat treatment status.

A heating element 5 made of a high frequency heater or a heating wire is installed in the hole 4 of the gear body 2 of the welding gear, and a power source 6 provides a heating element 5 . Further, a cooler 7 is installed around the teeth 1, and air or water is supplied from a supply port 71 and discharged from a discharge port 72. The welding gear initially uses such a heating means to heat the parts other than the welded part.

That is, the temperature of the mild steel part that constitutes the gear body 2 is 300°C.
Below qo, the temperature difference between the welded part 3 and the gear body 2 is 150
Rapidly heat to above ℃. For rapid heating while maintaining this temperature difference of 150° C. or more, water or air is supplied to the cooler 7 to lower the welded portion 3 to a low temperature. Next, the welded gear, in which a temperature gradient has occurred between the welded portion 3 and the gear body 2, is returned to room temperature. In this heating and cooling operation,
Initially, heating produces tensile stress, but subsequent cooling causes compressive stress to act as a reaction force.

Figures 3 and 4 schematically show the distribution of residual stress in the welded gear during this process.
The same parts as in the figure are given the same reference numerals. Figure 3 shows the stress distribution when the molten saddle remains, and Figure 4 shows the stress distribution when the parts other than the weld are heated to high temperature. where T indicates tensile stress and C indicates compressive stress.

Next, the temperature of the entire welded gear to which compressive residual stress has been applied by the above-described operation is gradually raised in a furnace to 60,000 to 6,500 C (recrystallization temperature), and held at this temperature for 3 to 4 hours.

In such a heat treatment method, the parts other than the weld are heated to a high temperature to avoid generating compressive stress in the weld, and the tensile residual stress generated during welding is thereby reduced to below the yield point, or the tensile residual stress is reduced to compressive residual stress. In this state, it is possible to pass through a temperature range of 300 to 55,000 degrees Celsius, where SR cracking occurs, making it possible to prevent SR cracking.

That is, by using the method of this example, SR
Since it is possible to eliminate residual stress in the weld and improve the material quality without causing cracks, it not only greatly improves the reliability of the product's strength, but also makes the heat treatment process simple and easy. can do.

Figure 5 shows another embodiment, in which the same parts as in Figures 1 and 2 are given the same reference numerals, and the difference from these figures is that heating of parts other than the welded part is In this case, the flame 81 of the gas burner 8 is used as the means, and the same effect can be obtained in this case as well.

Although Cr-Mo based low alloy steel was shown in the examples, SR cracking was observed in low manganese steel, silicon manganese steel, manganese chromium steel, manganese molybdenum steel, manganese chromium molybdenum steel, nickel and chromium molybdenum steel. Since this generally occurs in low alloy steels such as chromium-molybdenum steels, this heat treatment method can be similarly applied.

However, since SR cracking is particularly severe when applied to 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 the drawing]

Fig. 1 is a plan view showing the implementation status of one embodiment of the heat treatment method for welded joints of the present invention, Fig. 2 is a sectional view of the same, Fig. 3 is a residual stress distribution diagram of the welded gear as it is welded, and Fig. 4 The figure is a stress distribution diagram of a welded gear in one step of the heat treatment method for a welded joint of the present invention, and FIG. 5 is a sectional view showing the implementation situation of another embodiment. Welded part, 4
...Shaft hole, 5...Sparkling body, 7...Cooler, 8...Gas burner 1--. Tsutomu' figure Leopard 2 figure 3 figure Chi figure purple figure

Claims (1)

[Scope of Claims] 1. A heat treatment method for a welded joint for removing tensile residual stress generated in a joint made by welding low alloy steel and carbon steel and for improving the material quality, in which a portion of the joint other than the welded portion is provided. The joint is heated to create a temperature difference between the welded part and the welded joint that yields the tensile residual stress occurring in the welded joint on the tensile side, and then cooled to room temperature to eliminate the tensile residual stress generated during welding by stress reversal. An operation that changes the stress to compressive residual stress;
A method for heat treating a welded joint, comprising the steps of: gradually increasing the temperature of the entire welded joint to a recrystallization temperature; and holding the welded joint at the recrystallization temperature for a predetermined period of time. 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. 3. Heating other than the welded part is performed by rapid heating to 300°C or less, the temperature difference between the welded part and the welded part is 150°C or more, and the predetermined heat treatment temperature is 600°C to 650°C. A method for heat treating a welded joint according to claim 1 or 2.