CN103341695A - Method for improving mechanical property of hardened and tempered low-alloy high-strength steel GMAW connector - Google Patents

Abstract

The invention discloses a method for improving the mechanical property of a hardened and tempered low-alloy high-strength steel GMAW connector so as to solve the problems that when hardened and tempered low-alloy high-strength steel through quenching plus low temperature tempering is welded, tempering softening happens to one of heat affected zones of a welded joint, so that the quality of the welded joint is reduced. The method is characterized in that the texture of a softened area of the welded joint is improved with the help of the thermal cycle characteristic that the heating rate and cooling rate of laser are high ; laser remelting is conducted on the softened area of the welded joint to reduce the width of the softened area with the help of the characteristics that a welded line is deep, the aspect ratio is high, thermal input is low, a heat affected zone is small, and welding deformation is slight in laser welding deep penetration welding; therefore, the strength and toughness of the hardened and tempered low-alloy high-strength steel GMAW connector through quenching plus low temperature tempering can be improved.

Description

A Method for Improving the Mechanical Properties of Quenched and Tempered Low Alloy High Strength Steel GMAW Joints

【Technical field】

The invention belongs to the technical field of material processing and relates to a method for improving the mechanical properties of quenched and tempered low-alloy high-strength steel GMAW joints.

【Background technique】

Low-alloy high-strength steel is widely used in construction machinery, electric power, pressure vessels, automobiles and other fields due to its ultra-high strength, good ductility and good welding performance. The types of low-alloy high-strength steel can be divided into non-quenched and tempered steel and quenched and tempered steel. Quenched and tempered low-alloy high-strength steel is one of the fastest-growing and most dynamic steel types in recent decades, and its yield strength can reach 880 ～1176MPa, due to its ultra-high strength, good plastic toughness and weldability, it is not only widely used in bridges, buildings, ships, high-pressure vessels, construction machinery and vehicles, but also used in products with high strength requirements or Components, such as rocket motor casings, aircraft landing gear, etc.

Due to its excellent performance and remarkable economic benefits, quenched and tempered low-alloy high-strength steel is widely used in engineering welded structures, and how to obtain welded structural parts with excellent performance has always been the key research goal of researchers. For quenched and tempered low-alloy high-strength steel, in addition to the common problems of high-strength steel welding, welding cold cracks and welding heat-affected zone embrittlement, the softening of welded joints is also a key technical problem encountered in welding of this steel type. This is because quenched and tempered low-alloy high-strength steel is based on low-carbon steel by adding alloying elements that improve hardenability and then undergoes quenching and tempering (quenching + tempering) heat treatment to obtain low-carbon martensite with high strength and good toughness+ The lower bainite mixed structure, while welding, under the action of welding heat cycle, the heat-affected zone of the joint is heated to the area exceeding the quenching and tempering temperature, and there will be a softening zone whose strength and hardness are lower than that of the base metal. The area may become the weak area of the joint strength, that is, the softening problem of the welded joint occurs. After welding, if the martensite can be self-tempered during the slow cooling process, the occurrence of cold cracks can be alleviated; but if the cooling rate is fast, the martensite cannot self-temper, and its tendency to cold cracks will inevitably increase. Studies have shown that when low-alloy high-strength steel is welded in the quenched and tempered state, cracks and embrittlement can be controlled by preheating and post-weld heat preservation, while problems caused by softening cannot be solved after welding and need to be controlled in the welding process. For example, Wang Lianfang and others conducted welding research on DILLMAX965 quenched and tempered high-strength steel produced by Ukino Steel Company in France. The research shows that only when the heat input is strictly controlled to make t _8/5 <10s, the heat-affected zone in the welded joint does not exist. softening phenomenon. Zhao Yuzhen and others conducted welding research on SS400 steel, and studied the fine structure of the coarse-grained zone of the pulsed MAG (80%Ar+20%CO2) welded joint. There is a large amount of upper bainite structure, and when welding with lower heat input energy, the heat-affected zone is mainly the lower bainite structure with good toughness, and there is no softening phenomenon in the heat-affected zone. Summarizing the above methods, it can be seen that reducing the heat input as much as possible is an effective way to avoid softening of the heat-affected zone during the welding of quenched and tempered high-strength steel, but this method also brings limitations and lack of flexibility to the high-strength steel welding method. Therefore, It is necessary to find other effective methods to solve the softening problem of heat-affected zone of quenched and tempered low-alloy high-strength steel GMAW joints.

【Content of invention】

Aiming at the problem that tempering and softening of the HAZ (heat-affected zone) of the welded joint during GMAW welding of quenched and tempered low-alloy high-strength steel causes the joint strength to decrease, the present invention provides a method for improving the mechanical properties of the quenched and tempered low-alloy high-strength steel GMAW joint .

The present invention is realized through the following technical solutions:

A method for improving the mechanical properties of quenched and tempered low-alloy high-strength steel GMAW joints, comprising the following operations:

1) Measure and calibrate the position of the weld seam and fusion line of the welded joint, calculate and calibrate the position and width of the softened zone of the welded joint according to the maximum temperature calculation formula of the welding thermal cycle curve;

2) Use laser to remelt the softened zone of the welded joint;

3) Grinding the joint surface after laser remelting.

The softening zone refers to the zone in the heat-affected zone of the welded joint where the tensile strength is lower than that of the base metal.

When remelting the softening zone, one-sided or double-sided laser remelting can be used to remelt the softening zone of the butt joint.

Before measuring the softening zone of the calibration welded joint, the following processing is also included:

First, the surface of the sample joint to be laser remelted is polished and polished;

The welded joint is then corroded with a corrosive agent until the joint weld, fusion line, heat-affected zone, and various areas of the base metal appear.

The corrosive agent refers to the corrosive agent that can be used in metallographic corrosion analysis.

The GMAW joint is a welded joint formed by GMAW welding of a quenched + low-temperature tempered low-alloy high-strength steel sheet.

The thin plate refers to a steel plate with a thickness ranging from 0.2 to 6 mm.

The remelting of the joint softening zone refers to remelting the penetrating or non-penetrating softening zone of the joint.

The calculation of the position and width of the softened zone of the welded joint is:

The maximum temperature calculation formula based on the welding cycle curve:

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 4.13</span>}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}\mathrm{<span\; class="notranslate">\; \&delta;d</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula: E is the welding heat input; T ₀ is the initial temperature of the weldment; T _M is the melting point of the base metal, d is the distance from the fusion line; δ is the thickness of the base metal; c _v is the specific heat of the material at constant volume;

The distance from the position of the peak temperature T _max in the derived joint HAZ to the fusion line is

$\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>\frac{\mathrm{<span\; class="notranslate">\; E.</span>}}{\mathrm{<span\; class="notranslate">\; 4.13</span>}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Then the width of the softening zone in the joint HAZ is:

$\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; d</span>}}_{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; d</span>}}_{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>\frac{\mathrm{<span\; class="notranslate">\; E.</span>}}{\mathrm{<span\; class="notranslate">\; 4.13</span>}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

为HAZ中峰值温度为回火温度的位置与熔合线距离；为HAZ中峰值温度A_c1位置与熔合线距离。In the formula, b is the width of HAZ softening zone; T _h is the self-tempering temperature of material controlled cooling production;

is the distance between the position where the peak temperature is the tempering temperature and the fusion line in the HAZ; is the distance between the position of the peak temperature A _c1 and the fusion line in the HAZ.

Compared with the prior art, the present invention has the following beneficial technical effects:

The method for improving the mechanical properties of quenched and tempered low-alloy high-strength steel GMAW joints provided by the present invention is a method based on laser remelting to improve the mechanical properties of the softened zone of quenched and tempered low-alloy high-strength steel sheet GMAW joints. The thermal cycle characteristics of fast heating and cooling speed improve the structure of the HAZ softening zone of the GMAW joint of the quenched + low-temperature tempered low-alloy high-strength steel sheet; deep penetration welding with the help of laser welding has deep weld seam, high aspect ratio, and small heat input heat-affected zone Small and small welding deformation. Laser remelting of GMAW joints of quenched + low-temperature tempered low-alloy high-strength steel sheets can reduce the width of the HAZ softening zone of the joints, thereby improving the strength and toughness of the joints.

【Description of drawings】

Figure 1 is a schematic diagram of the calibration of the joint area and the position of the softening area;

Figure 2 is a schematic diagram of the joint laser remelting scheme;

Figure 3 is a schematic diagram of the macroscopic morphology of the joint after laser remelting;

The physical picture of the joint after the laser remelting of the embodiment in Fig. 4;

Figure 5 shows the stress-strain curves of the joint tensile specimen before and after laser remelting.

【Detailed ways】

The present invention will be further described in detail below in conjunction with specific embodiments.

A method for improving the mechanical properties of quenched and tempered low-alloy high-strength steel GMAW joints provided by the invention comprises the following operations:

1) Measure and calibrate the position of the weld seam and fusion line of the welded joint, calculate and calibrate the position and width of the softened zone of the welded joint according to the maximum temperature calculation formula of the welding thermal cycle curve;

2) Use laser to remelt the softened zone of the welded joint;

3) Grinding the joint surface after laser remelting.

Specifically, the following treatments are also included before measuring the softening zone of the calibration welded joint:

First, the surface of the sample joint to be laser remelted is polished and polished;

The welded joint is then corroded with a corrosive agent until the joint weld, fusion line, heat-affected zone, and various areas of the base metal appear. The corrosive agent refers to the corrosive agent that can be used in metallographic corrosion analysis.

The softening zone refers to the zone in the heat-affected zone of the welded joint where the tensile strength is lower than that of the base metal. When remelting the softening zone, single-sided or double-sided laser remelting can be used to achieve remelting of the softened zone of the joint. The remelting of the joint softening zone refers to remelting the penetrating or non-penetrating softening zone of the joint.

Specific examples are given below for illustration.

In this example, a JHM-1GXY-400X Nd:YAG laser is used to remelt the HAZ softening zone of the 1.5mm thick 1000MPa fine-grained high-strength steel 20MnTiB GMAW joint. The specific implementation steps include:

(1) GMAW is used to weld 20MnTiB butt joints, the shielding gas is argon-rich mixed gas, and the line energy is 9.5J/cm. The sample to be laser remelted is cut by wire cutting, and the specific size is 200mm×30mm×1.5mm.

(2) Grinding and polishing the sample joints to be laser remelted.

(3) Corrode the 20MnTiB GMAW welded joint area with 4% nitric alcohol solution (volume fraction) on the polished sample until the joint weld, fusion line, heat-affected zone and various areas of the base metal appear,

(4) Combined with metallographic observation and calibration, measure and calibrate the weld seam, fusion line, and heat-affected zone of 20MnTiB GMAW joints, calculate and calibrate the position and width of the softening zone of the welded joint according to the maximum temperature calculation formula of the welding thermal cycle curve, the specific steps are as follows :

The formula for calculating the maximum temperature from the welding thermal cycle curve:

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 4.13</span>}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}\mathrm{<span\; class="notranslate">\; \&delta;d</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

The distance from the position of the peak temperature T _max in the derived joint HAZ to the fusion line is

$\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>\frac{\mathrm{<span\; class="notranslate">\; E.</span>}}{\mathrm{<span\; class="notranslate">\; 4.13</span>}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Then the width of the softening zone in the joint HAZ is

$\mathrm{<span\; class="notranslate">\; b</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; d</span>}}_{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; d</span>}}_{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>\frac{\mathrm{<span\; class="notranslate">\; E.</span>}}{\mathrm{<span\; class="notranslate">\; 4.13</span>}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; v</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

In the embodiment: the heat input E is 8.2kJ/cm; the initial temperature T ₀ of the weldment is 20°C; the melting point T _M of 20MnTiB is 1530°C; the thickness δ of the base metal is 6mm; the specific heat c _v of 20MnTiB is 6.7J/ (cm ³ ℃); 20MnTiB controlled cooling production tempering temperature T _h is 200 ℃; then it can be seen from the formula (2) that the distance between the position where the peak temperature in the joint HAZ is the tempering temperature and the fusion line d _Tmax = _Th is 4mm; The distance d _Tmax =A _cl between the position of the peak temperature A _c1 and the fusion line in the joint HAZ is 2.5 mm. The schematic diagram is Figure 1.

(5) Use JHM-1GXY-400X Nd:YAG laser to remelt the calibrated 20MnTiBGMAW joint HAZ softening zone;

The specific plan is: ① use pulsed laser to remelt the softened zone of the GMAW joint of 20MnTiB, the schematic diagram is shown in Figure 2;

②In order to reduce heat input, avoid wide laser heat-affected zone and small welding deformation, the GMAW joint of 20MnTiB is welded with two small heat input up and down, so as to achieve remelting of the HAZ softening zone of the joint, The schematic diagram is Fig. 3, and Fig. 4 is the physical figure of the embodiment;

(6) Grinding the joints after laser remelting.

The mechanical performance test of the example sample is carried out by the joint tensile performance test. Figure 5 is the stress-strain curve obtained by the joint tensile test of the example. The black curve in the figure is the joint that has not passed through the laser remelting softening zone Tensile stress-strain curve, the curve shows that the tensile strength of the joint is 766MPa, the red curve in the figure is the tensile stress-strain curve of the joint after the laser remelting softening zone, the curve shows that the tensile strength of the joint is 906MPa, from the two The comparison of the two curves shows that laser remelting can significantly improve the mechanical properties of the softened zone of the GMAW joint of 20MnTiB.

Finally, it is noted that this embodiment is only used to illustrate the technical solution of the present invention without limitation. Other modifications or equivalent replacements made by those of ordinary skill in the art to the technical solution of the present invention, as long as they do not depart from the spirit of the technical solution of the present invention, All should be included in the scope of claims of the present invention.

Claims (9)

1. a method of improving modified low-alloy high-strength steel GMAW joint mechanical property is characterized in that, comprises following operation:

1) measures and demarcate the weld seam of welding point, the position of melt run, calculate position and the width that formula calculated and demarcated the welding point softened zone according to the maximum temperature of welding thermal cycle curve;

2) adopt laser that the softened zone of welding point is carried out remelting;

3) joint surface behind the laser remolten is carried out grinding process.

2. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 1 joint mechanical property is characterized in that, described softened zone refers to take place in the welding point heat affected area zone that tensile strength is lower than mother metal tensile strength.

3. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 1 joint mechanical property is characterized in that, when the softened zone was carried out remelting, the mode of employing single face or two-sided laser remolten reached the remelting of butt joint softened zone.

4. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 1 joint mechanical property is characterized in that, also comprises following processing before measuring demarcation welding point softened zone:

The sample joint surface that at first will treat laser remolten carries out sanding and polishing;

Adopt corrosive agent to corrode to welding point then, until each zone that shows joint welding, melt run, heat affected area and mother metal.

5. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 4 joint mechanical property is characterized in that, described corrosive agent refers to can be used for the corrosive agent of metal metallographic corrosion analysis.

6. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 1 joint mechanical property is characterized in that, described GMAW joint welds formed welding point for the modified low-alloy high-strength steel thin plate of quenching+lonneal GMAW.

7. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 6 joint mechanical property is characterized in that, described thin plate refers to that thickness range is the steel plate of 0.2～6mm.

8. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 1 joint mechanical property is characterized in that, the remelting of described butt joint softened zone refers to butt joint softened zone penetration or the capable remelting of non-penetration.

9. the method for the modified low-alloy high-strength steel GMAW of improvement as claimed in claim 1 joint mechanical property is characterized in that, being calculated as of the position of welding point softened zone and width:

Maximum temperature according to the weld cycle curve is calculated formula:

$\frac{1}{T_{\max }-T_0}=\frac{4.13c_v\mathrm{\&delta;d}}{E}+\frac{1}{T_M-T_0}---\left(1\right)$

In the formula: E is the sweating heat input; T ₀Be the weldment initial temperature; T _MBe the mother metal fusing point, d is the distance of leaving melt run; δ is mother metal thickness; c _vBe the material specific heat at constant volume;

Derive peak temperature T among the joint HAZ _MaxThe melt run distance is left in the position

$d=(\frac{1}{T_{\max }-T_0}-\frac{1}{T_M-T_0})\frac{E}{4.13c_v\mathrm{\&delta;}}---\left(2\right)$

Then the width of softened zone is among the joint HAZ:

$b=d_{T_{\max }=T_h}-d_{T_{\max }=A_{c1}}=(\frac{1}{T_h-T_0}-\frac{1}{A_{c1}-T_0})\frac{E}{4.13c_v\mathrm{\&delta;}}---\left(3\right)$

B is HAZ softened zone width in the formula; T _hBe the cold production self tempering temperature of material control; d _Tmax=T _hBe position and the melt run distance of temperature for peak temperature among the HAZ; d _Tmax=A _ClBe peak temperature A among the HAZ _C1Position and melt run distance.

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