KR20110074136A - Post weld heat treatment of high carbon steeel - Google Patents

Abstract

PURPOSE: A method of processing the rear of a welding part of high carbon steel is provided to enhance quality and productivity of a welding part of high carbon steel since in welding high carbon steel, the low-temperature crack is decreased and fracture strength can be increased, by quick rear processing. CONSTITUTION: A method of processing the rear of a welding part of high carbon steel comprises next steps. A welding part is maintained at a temperature of below TL((Ms+Mf)/2), after which the welding part is heated to a temperature of 750~850°C. The temperature of the welding part is maintained for below 10sec after the heating. The welding part is cooled. When being maintained at the temperature of below TL((Ms+Mf)/2), a period of time is below 1hour. The carbon content of the high carbon steel is 0.5~0.8weight%.

Description

Post-heat Treatment Method for High Carbon Steel Welded Joints {POST WELD HEAT TREATMENT OF HIGH CARBON STEEEL}

The present invention relates to a weld post-heat treatment method capable of reducing cracks generated in the weld after welding high carbon steel (0.5 to 0.8 C) and having a higher breaking strength.

High carbon steel is a steel that improves strength and wear resistance of materials by increasing carbon content in steel and by special heat treatment, and is mainly used as mechanical structural steel, tool steel, and wear resistant steel. However, such steels are known to have a very low temperature crack crack at the weld due to a high carbon content or a high carbon equivalent (Ceq) on the alloy component, and thus also have a low breaking strength.

Due to such poor weld characteristics, welding of products using high carbon steel is required for a long time preheating and post-heat treatment at very high temperatures, and this causes a problem of deterioration of weld productivity and weld stability.

In order to solve this problem, in Japanese Laid-Open Patent Publication Nos. 1993-132719 and 2004-076159, laser welding is applied, and in Japanese Laid-Open Patent Publication No. 2000-317642, when flash butt welding is applied, in line welding of high carbon steel A technique for reducing weld cracks is disclosed by making the resulting martensite hardened structure into a tempered martensite structure by post-heat treatment,

Japanese Laid-Open Patent Publication No. 1996-215872 discloses a method of controlling the phase transformation of a laser welded portion through a pre / post heat treatment to guide the welded tissue to a pearlite structure.

However, this method is easy in the case of thin plate of less than 3mm applied material, pre-heat treatment of the welded part, but the thicker the thickness of the applied steel, there is a problem such as the increase in the size of pre-heat treatment equipment and increase the heat treatment time, Particularly, in the high carbon steel of a component system having a relatively fast ferrite or pearlite phase transformation rate, phase transformation control through pre / post heat treatment was possible, but it is difficult to apply when the ferrite or pearlite phase transformation is late.

One aspect of the present invention relates to a post-heat treatment method of a high carbon steel welded part to reduce cracks in the welded part and to have excellent breaking strength when welding high carbon steel having a carbon content of 0.5 to 0.8 wt%.

The present invention is a method of post-heat treatment after welding,

Maintaining the welded weld below the T _L ((M _s + M _f ) / 2) temperature;

Heating to a temperature (T _P ) of 750-850 ° C. after the holding;

Holding for a holding time D _P of 10 seconds or less after the heating; And

It provides a high carbon steel welded post-heat treatment method comprising the step of cooling.

According to the present invention, in welding high carbon steel, even after a short time post-treatment, the low temperature crack of the weld can be reduced and the breaking strength can be increased, thereby improving the quality and weld productivity of the high carbon steel weld.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present inventors have studied in order to reduce the crack of the weld portion and to secure a higher breaking strength when welding high carbon steel (0.5 ~ 0.8C),

After welding, the martensite structure generated in the weld part is heated within a short time to generate fine austenite in the martensite, and then cooled to allow the austenite to remain, thereby recognizing that the hardenability of the weld structure can be reduced. By devising a method for controlling the microstructure of the weld through heat treatment, the present invention has been achieved.

Welding method applied to the welding of high carbon steel in the present invention is not limited, and typically there are laser welding, electron beam welding, arc welding, resistance welding, friction stir welding.

After heat treatment method of the present invention as shown in FIG. Welding after, the welding welds predetermined temperature (T _L) less than a predetermined time (D _L) maintaining the step of heating at a constant temperature (T _P), after said heating step to maintain a certain period of time (D _P) and cooled in an It comprises a step.

The present invention is a method of post-heat treatment after welding the high carbon steel, the welded portion after welding the high carbon steel is maintained below the T _L temperature. The T _L temperature is defined as (M _s + M _f ) / 2. This is because when the T _L temperature is exceeded, sufficient residual austenite is not produced during post-heating and cooling.

In the present invention, M _s is a martensite starting temperature (martensite starting temperature), the M _f is a martensite transformation temperature (martensite finishing temperature) after the martensite transformation is complete. The M _s , M _f temperature depends on the steel composition.

In the present invention, the holding time D _L is a time required for the formation of martensite at the T _L temperature, and is not particularly limited thereto. However, since the risk of delayed fracture may occur at the weld after an excessive time, it is preferable to set the maximum time to 1 hour or less, and more preferably 1 minute or less in consideration of work efficiency and time economics.

After maintaining the welded portion, it is heated to a constant temperature. At this time, the heating temperature (T _p ) is preferably 750 ~ 850 ℃. When the T _P temperature is less than 750 ° C., the austenite transformation is not sufficient in martensite for residual austenite production. When the T _P temperature is higher than 850 ° C., too much austenite is produced, and when the cooling is performed, the transformation into martensite is performed. There is a risk of increasing curability.

After the heating, it is maintained for a certain time. At this time, the holding time D _p is preferably 10 seconds or less. This is because when the holding time exceeds 10 seconds, the effect required by the present invention cannot be achieved due to the reduction of the retained austenite fraction and the transformation into martensite during cooling due to excessive generation of austenite at high temperature.

Figure 2 shows a schematic diagram of the post-welding post-heat treatment method in the present invention. In FIG. 2, (1) shows a base material, (2) shows a welding machine, and (3) shows a post heat treatment machine.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following examples.

(Example)

In this embodiment, the welding was performed on high carbon steel of 0.75C-1Si-1.5Mn-1.8Al, and the post-heat treatment was performed under the post-heat treatment conditions shown in Table 1 below. The form was measured and the results are shown in Table 1 below. M _s temperature of the test piece was 220 ℃, M _f temperature is 95 ℃.

In Table 1, the breaking strength and the weld hardness were measured by the Tensile Restraint Cracking (TRC) test for the evaluation of the reproducing weld low temperature crack sensitivity. That is, the weld portion and the post-heat treatment welding heat cycle were simulated, and the breaking strength was measured by adding a tensile stress of a constant strain rate at 250 ° C. Hardness was measured on broken specimens.

division Post heat treatment T _L
(℃) D _L
(sec) T _P
(℃) D _P
(sec) Breaking strength
(MPa) Weldment Hardness
(Hv0.5) Wave form Comparative Example 1 none - - - - 574 850 Grain boundary Comparative Example 2 has exist 300 One 800 One 630 750 Grain boundary Comparative Example 3 has exist 250 One 800 One 614 735 Grain boundary Comparative Example 4 has exist 150 One 900 One 648 780 Grain boundary Comparative Example 5 has exist 150 One 700 One 627 487 Grain boundary Comparative Example 6 has exist 150 One 800 20 680 470 Grain boundary Inventive Example 1 has exist 150 One 800 One 962 330 halitosis Inventive Example 2 has exist 100 One 800 One 997 323 halitosis Inventive Example 3 has exist 100 One 800 5 955 350 halitosis Honorable 4 has exist 120 2 760 2 985 334 halitosis Inventory 5 has exist 120 3 840 3 1022 317 halitosis Inventory 6 has exist 120 30 800 3 966 348 halitosis

The comparative example is a conventional case, in which a weld portion without post-heat treatment is reproduced. In this case, the breaking strength is about 574 MPa, and the weld portion hardness is about 920 Hv, and the wave form is also a brittle fracture shape. Is showing.

On the other hand, in the case of Inventive Examples 1 to 6 satisfying the conditions of the present invention, the breaking strength is close to 1000 MPa, and the weld hardness is also lowered to 350 Hv or less, which shows that the weld characteristics are excellent. In addition, the hardness value is 370 Hv or less of the maximum hardness limit value of the welded portion of the structural steel (Road Bridge Standard Specification, 2005) it can be confirmed that much improved in terms of structure stability.

Inventive Examples 1 to 6, unlike the comparative example, the grain boundary fracture shape disappeared and exhibited intragranular fracture. On the other hand, Comparative Examples 2 to 6, which are outside the scope of the present invention, still exhibit brittle fracture-like grain boundary fracture shape even after the post-heat treatment, and have no significant effect in terms of breaking strength and lowering of hardness.

1 is a graph of time and temperature showing the post-heat treatment method of the present invention.

2 is a schematic diagram of a post-heat treatment method applied to the present invention.

Figure 3 is a graph showing the Tensile Restraint Cracking (TRC) test for the evaluation of cold cracking susceptibility of reproducing weld.

Claims (4)

In the post-heat treatment method after welding, Maintaining the welded weld below the temperature of T _L ((M _s + M _f ) / 2); Heating to a temperature (T _P ) of 750-850 ° C. after the holding; Holding for a holding time D _P of 10 seconds or less after the heating; And Cooling step High carbon steel welded post-heat treatment method comprising a. (M _s : temperature at which martensite transformation starts to occur, M _f : temperature at which martensite transformation ends and all become martensite tissues, and M _s and M _f temperatures differ depending on the steel grade) The method according to claim 1, When the temperature is maintained below the temperature T _L ((M _s + M _f ) / 2), the holding time D _L is less than 1 hour. The method according to claim 1, The high carbon steel is a carbon content of 0.5 ~ 0.8% by weight of the high carbon steel welded post-heat treatment method. The method according to claim 1, The welding is any one of laser welding, electron beam welding, arc welding, resistance welding and friction stir welding.

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