A single-side welding and hardening process for DQ wear-resistant steel thin plates

Abstract

The invention belongs to the field of welding technology, and discloses a single-sided welding hardening process for thin plates in DQ wear-resistant steel: 1) the parent material is DQ wear-resistant steel with a thickness of 3 to 8 mm; 2) the hardness of the weld and the heat-affected zone is estimated, and the welding wire composition and welding process parameters are adjusted to make the estimated hardness not less than 87% of the hardness of the parent material; 3) single-sided gas shielded welding is adopted, and water cooling is performed when the overheating zone and the normalizing zone of the welding heat-affected zone are completely austenitized and the incomplete normalizing zone is partially austenitized, and water is avoided from directly contacting the arc during water cooling, and water is added to the welding part without the arc. After the arc, the cooling water covers the welding part by itself and starts to cool quickly to form a hardened structure. The invention estimates the hardness of the weld and the heat-affected zone, selects the welding wire and the welding process, grasps the opportunity of austenitization of the heat-affected zone during welding, and quickly cools and quenches to form a hardened structure, so that the hardness is significantly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of welding, and in particular relates to a single-side welding hardening process for a DQ wear-resistant steel medium-thin plate. Background Art

[0002] With the continuous development of economic construction, more and more construction machinery is used, and the demand for wear-resistant steel plates is growing rapidly. Green and reduced-volume steel manufacturing technology has promoted the development of direct quenching (DQ) low-alloy wear-resistant steel. Due to its low alloy content and good comprehensive performance, it has gradually been increasingly used in loaders, excavators, dredging pipes, etc., and mainly in medium and thin plates. Through online direct quenching and low-temperature tempering process, the steel grade can achieve high strength and high toughness, and has excellent welding crack resistance.

[0003] Wear-resistant steel is generally welded to other supporting structures and used in wear-resistant parts. The welds mostly play a connecting role. However, in some occasions, such as the inner plate of the box, the welds are also required to have good wear resistance. For such applications, the welds not only play a connecting role, but the welded joints have become working parts, so there are considerable requirements for the wear resistance of the welded joints. However, since the alloy content of DQ wear-resistant steel is significantly reduced compared with traditional steel, if conventional welding technology is used, the hardness and wear resistance of the welded joints will be significantly reduced, becoming a weak link.

[0004] The document "WNM360L wear-resistant steel material welding process development and application, Metal Processing, 2018.8" introduces the wear-resistant tail drum shell for ships. The shell manufacturing process involves butt welding. 40mm thick WNM360L wear-resistant steel is used, and the hardness of the parent material is 293~325HV 10 , the hardness of the heat affected zone is 214~229HV 10 The hardness of the weld is 219~229HV 10 , the hardness of the welded joint is significantly reduced. In the literature "Organization and Properties of NR360 Bainite Wear-Resistant Steel Plate, Journal of Anhui University of Technology, 2009.7", the welding performance of 30mm thick NR360 bainite wear-resistant steel plate is studied. The parent material structure is lamellar carbide-free bainite and residual austenite structure, with a hardness of 355HB; the weld structure is mainly blocky and acicular ferrite structure, with a hardness as low as 190HV.

[0005] Therefore, how to solve the problem of significantly reduced hardness of welded joints is a difficult problem that needs to be solved urgently in the industry. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a single-sided welding hardening process for thin plates in DQ wear-resistant steel in view of the shortcomings of the prior art. By estimating the hardness of the weld and the heat-affected zone, selecting welding wire and welding process, the heat-affected zone is austenitized at the right time during welding to quickly cool and quench, thereby forming a hardened structure and significantly improving the hardness.

[0007] In order to solve the technical problem proposed by the present invention, the present invention provides a single-sided welding hardening process for a thin plate of DQ wear-resistant steel, comprising the following steps:

[0008] 1) Parent material properties: DQ wear-resistant steel, carbon equivalent CE is 0.42-0.48%, thickness is 3-8mm, hardness is HV 10 ≥330, -20℃ impact energy KV2≥27J, tensile strength Rm≥1100Mpa;

[0009] 2) Selection of welding wire and welding process: Estimate the hardness of the weld and heat-affected zone according to the following formula, adjust the welding wire composition and welding process parameters so that the estimated hardness is not less than 87% of the hardness of the parent material;

[0010] HV=0.75HV1+0.25HV2

[0011] HV1=97+949C+27Si+11Mn+8Ni+16Cr+20lgv r

[0012] HV2=-348+185C+330Si+153Mn+66Ni+144Cr+191Mo+

[0013] lgv r (89+54C-55Si-22Mn-10Ni-20Cr-33Mo)

[0014]

[0015] Where: HV is the estimated hardness, HV1 is the martensite hardness, and HV2 is the bainite hardness;

[0016] C, Si, Mn, Ni, Cr, Mo are the weight percentages of the elements in the welding wire or base material, in %; the welding wire composition is used to calculate the weld hardness, and the base material composition is used to calculate the heat-affected zone hardness;

[0017] Vr is the characteristic temperature cooling rate, in K / h; T is the characteristic temperature, T0 is the preheating temperature, q is the welding power, v is the welding speed, and λ is the thermal conductivity of steel;

[0018] 3) Welding: Use single-sided gas shielded welding. When the overheating zone and normalizing zone of the welding heat affected zone are completely austenitized and the incomplete normalizing zone is partially austenitized, water cooling is performed. Avoid direct contact between water and arc during water cooling. Add water to the welding area without arc. After the arc, the cooling water will cover the welding area by itself and start to cool quickly to form a hardened structure.

[0019] In the above scheme, the chemical composition and weight percentage of the parent material are: C: 0.15-0.25%, Si: 0.4-0.7%, Mn: 1.2-1.5%, P≤0.02%, S≤0.015%, Cr: 0.1-0.3%, Als: 0.02-0.05%, Ti≤0.03%, and the rest is Fe and unavoidable inclusions.

[0020] In the above scheme, the chemical composition and weight percentage of the welding wire are: C: 0.06-0.11%, Si: 0.5-0.7%, Mn: 1.5-1.8%, P≤0.02%, S≤0.015%, Ni: 0.8-1.2%, Cr: 0.3-0.5%, Cu: 0.15-0.3%, Ti: 0.06-0.15%, and the rest is Fe and unavoidable inclusions.

[0021] In the above scheme, the characteristic temperature cooling rate does not exceed the maximum cooling rate of room temperature water quenching of 200°C / s.

[0022] In the above scheme, the welding power is equal to the welding current multiplied by the welding voltage, the welding current is 180-290A, the welding voltage is 20-29V, and the welding speed is 38-70cm / min.

[0023] In the above scheme, the welding line energy of the welding wire is E=(1-2.5)×t, where the unit of E is kJ / cm, and t is the thickness of the base material in mm.

[0024] In the above scheme, the shielding gas for the single-sided gas shielded welding is a mixed gas of 78-82% Ar and 18-22% CO2, and the shielding gas flow rate is 10-20 L / min.

[0025] In the above scheme, the water flow rate of the water cooling is 6 to 12 L / min, the water temperature is room temperature, the water cooling width is 30 to 60 mm, and the cooling rate is 75 to 200°C / s.

[0026] In the above solution, the hardened structure is at least one of martensite and bainite.

[0027] Furthermore, the hardened structures formed in the weld, overheating zone and normalizing zone are martensite and bainite, of which the volume proportion of martensite is 60-90%; the hardened structures formed in the incomplete normalizing zone are martensite, ferrite and pearlite, of which the volume proportion of martensite is 30-60%.

[0028] The technical concept of the present invention is:

[0029] When welding steel, there will be welds, overheating zones, normalizing zones, incomplete normalizing zones, tempering zones between Ac1 and tempering temperature, and areas below tempering temperature. Overheating zone: also known as coarse grain zone, the welding temperature is above 1100℃, it is completely austenitized and the austenite grains grow significantly. Depending on the cooling rate after welding, it will be transformed into martensite, bainite, pearlite and ferrite, etc. Normalizing zone: also known as fine grain zone, the welding temperature is 900-1100℃, it is completely austenitized but the grains are fine. Depending on the cooling rate after welding, it will be transformed into martensite, bainite, pearlite and ferrite, etc. Incomplete normalizing zone: the welding temperature is 700-900℃, part of it is austenitized, and part of the parent material is tempered to precipitate carbides. Depending on the cooling rate after welding, the austenite will be transformed into martensite, bainite, pearlite and ferrite, etc. Tempering zone: includes tempered troostite, tempered martensite, or tempered martensite, depending on the tempering temperature of the steel plate itself.

[0030] For medium and thin plates, under normal welding conditions, due to the slow cooling after welding or the influence of tempering, it is difficult for these areas to form martensite again, so the hardness will drop significantly. If we try to quickly cool and quench the overheating zone, normalizing zone and incomplete normalizing zone immediately after austenitization during welding, these areas can obtain quenched martensite or bainite. At the same time, due to the effect of rapid cooling, the width of the tempering zone is also suppressed to a certain extent, so that the overall hardness of the welded joint is significantly improved. However, quenching during welding is not easy, and there is no public report on such technology. Therefore, in order to achieve this goal, after theoretical analysis, scheme design and welding experiments, the welding quenching process was successfully realized and the expected effect was achieved.

[0031] The welding quenching process of the present invention is not only very critical for the selection of base materials, welding wires and welding processes, but also particularly important for the control of the water cooling process. The austenitization time of the heat-affected zone during welding is extremely short, and the timing must be accurately grasped to start rapid cooling when it is still austenite, otherwise the quenching effect will not be achieved. Since the presence of water during welding can easily cause pores and cold cracks, which will affect the stability of the welding process, water cannot touch the arc side during water cooling. When water is added to the welding part without arc, the heat-affected zone near the molten pool can still be austenitized under the action of high temperature; after the arc, the water covers these places again, so that they are quickly cooled in time to produce a quenching effect; at the same time, due to the restraining effect of water cooling, the welding of medium and thin plates is like the welding of thick plates, and the heat-affected zone is greatly reduced. In addition, the water pressure cannot be too large. On the one hand, it may deform the welding part that is at high temperature or even molten, and on the other hand, it will affect the austenitization of the heat-affected zone in the thick direction of the arc, and the purpose of quenching and hardening cannot be achieved.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] In order to solve the problem that the hardness of DQ wear-resistant steel welded joints is significantly reduced, the present invention develops a welding and quenching process for DQ wear-resistant steel medium and thin plates with a thickness of 3 to 8 mm. By estimating the hardness of the weld and the heat-affected zone, selecting welding wire and welding process, grasping the opportunity of austenitization of the heat-affected zone during welding, and quickly cooling and quenching, the weld and the austenitized welding heat-affected zone form a hardened structure, and the hardness thereof is significantly improved compared with that of ordinary welding. The hardness of other parts of the heat-affected zone is also significantly increased due to the increase in the cooling rate after welding. At the same time, the width of the welding heat-affected zone is significantly reduced due to the restraint of water cooling. Therefore, the hardness of the welded joint is significantly improved compared with that of ordinary welding, and the hardness of most areas of the welded joint reaches more than 87% of the parent material, and has good application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a macroscopic morphology of the weld in Example 2 of the present invention.

[0035] Figure 2 This is the metallographic structure diagram of the welding overheating zone of Example 2 of the present invention. DETAILED DESCRIPTION

[0036] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0037] Examples 1-6

[0038] A single-side welding hardening process for DQ wear-resistant steel thin plates comprises the following steps:

[0039] 1) Parent material properties: DQ wear-resistant steel, chemical composition see Table 1, main properties see Table 2;

[0040] Table 1 Chemical composition of base material (wt%)

[0041] Base Material C Si Mn P S Cr Als Ti CE A 0.15 0.5 1.3 0.006 0.003 0.25 0.045 0.015 0.42 B 0.15 0.5 1.3 0.006 0.003 0.25 0.045 0.015 0.42 C 0.19 0.7 1.4 0.01 0.004 0.16 0.032 0.020 0.46 D 0.19 0.7 1.4 0.01 0.004 0.16 0.032 0.020 0.46

[0042] Note: The steel contains appropriate amount of Als and Ti. After finishing rolling, it is rapidly cooled and coiled at 270℃ for self-tempering. The tempered structure is martensite.

[0043] Table 2 Main properties of base material

[0044] Base Material Thickness(mm) <![CDATA[Hardness HV 10 > <![CDATA[-20℃KV2(J)]]> Rm(MPa) A 5 350 63 1120 B 8 355 63 1120 C 3 360 56 1150 D 6 353 56 1150

[0045] 2) Selection of welding wire and welding process: Estimate the hardness of the weld and heat-affected zone according to the following formula, adjust the welding wire composition and welding process parameters so that the estimated hardness is not less than 87% of the hardness of the parent material;

[0046] HV=0.75HV1+0.25HV2

[0047] HV1=97+949C+27Si+11Mn+8Ni+16Cr+20lgv r

[0048] HV2=-348+185C+330Si+153Mn+66Ni+144Cr+191Mo+

[0049] lgv r (89+54C-55Si-22Mn-10Ni-20Cr-33Mo)

[0050]

[0051] Where: HV is the estimated hardness, HV1 is the martensite hardness, and HV2 is the bainite hardness;

[0052] C, Si, Mn, Ni, Cr, Mo are the weight percentages of the elements in the welding wire or base material, in %; the welding wire composition is used to calculate the weld hardness, and the base material composition is used to calculate the heat-affected zone hardness;

[0053] Vr is the characteristic temperature cooling rate, in K / h, and shall not exceed the maximum cooling rate of 200℃ / s at room temperature water quenching; T is the characteristic temperature, which is 700℃; T0 is the preheating temperature, which is 25℃ at room temperature; q is the welding power, which is equal to the welding current multiplied by the welding voltage; v is the welding speed, and λ is the thermal conductivity of steel, which is 42W / (m·℃);

[0054] Table 3 Chemical composition of welding wire (wt%)

[0055] Welding wire C Si Mn P S Ni Cr Cu Ti a 0.08 0.60 1.76 0.012 0.007 0.9 0.35 0.2 0.10 b 0.087 0.53 1.68 0.014 0.004 1.0 0.32 0.2 0.09

[0056] Table 4 Welding process parameters

[0057]

[0058]

[0059] 3) Welding: Use single-sided gas shielded welding, the shielding gas is a mixture of 78-82% Ar and 18-22% CO2. When the overheating zone and normalizing zone of the welding heat affected zone are completely austenitized, and the incomplete normalizing zone is partially austenitized, water cooling is performed. During water cooling, avoid direct contact between water and arc, add water to the welding part without arc, and after the arc, the cooling water will cover the welding part by itself and start to cool quickly to form a hardened structure.

[0060] Table 5 Welding and water cooling process parameters

[0061]

[0062] Table 6 Measured hardness of welded joints

[0063]

[0064] Note: 1) R is the ratio of the width / plate thickness where the hardness of one side of the weld joint is lower than 87% of the base material; 2) The quenching zone includes the overheating zone and the normalizing zone, and the semi-quenching zone is the incomplete normalizing zone.

[0065] Examples 1 to 6 used welding wire (Table 3) and welding quenching process (Table 4 to 5) for the corresponding steel plates (Table 1 to 2). The welds were well formed without defects. The typical weld macroscopic morphology is as follows: Figure 1 As shown in Table 6, the hardness of different parts of the welded joint and the ratio R of the width of the 87% hardness zone on one side to the plate thickness are within the normal range. In order to control the hardness of the welded joint, the embodiment must keep the welding wire, welding process and parent material matched. Generally speaking, the welding hardness increases with the decrease of wire energy, the increase of alloy element content (or carbon equivalent) and the increase of plate thickness; further, the influence of these factors is comprehensively expressed by the relationship described above, which can be predicted by calculation, and the welding conditions can be optimized according to the prediction results.

[0066] Example 2 is used as an example for explanation: the thickness of the steel plate is 5 mm. Before formally determining the parameters, the above formula is used for overall pre-control. When using water-cooled welding, avoid direct contact between water and the arc. During water cooling, the weld and the heat-affected zone in the thickness direction are austenitized due to the "film boiling" and "bubble boiling" effects. After the arc, the cooling water covers the weld by itself and begins to cool rapidly, forming a hardening effect. The microstructures of the weld, overheating zone and normalizing zone are martensite and bainite, among which the microstructure of the overheating zone is as follows Figure 2As shown. The microstructure of the incomplete normalizing zone is martensite, ferrite and pearlite. The volume ratio of martensite in each zone is: 82% in the weld, 86% in the overheating zone, 65% in the normalizing zone, and 45% in the incomplete normalizing zone. The hardness of the heat-affected zone and the fully quenched zone reaches 350HV 10 , the non-complete quenching zone reaches 345HV 10 , which is basically equivalent to the parent material; the tempering zone is 277HV 10 However, due to the inhibition of water cooling, the tempering zone with low hardness is relatively narrow. The width / plate thickness ratio of the welded joint where the hardness on one side is lower than 87% of the base material is 0.6.

[0067] In comparative example 1, E / t reached 2.5 during welding, and the heat-affected zone at the bottom of the weld was a coarse-grained zone and a fine-grained zone, all of which were austenitized. However, since no water cooling was performed during welding, the cooling speed of this part was slow, forming a bainite-based structure, and its hardness did not reach the expected result.

[0068] In comparative example 2, the E / t is only 0.8 during welding, and the temperature of the heat-affected zone near the bottom of the weld has not reached the austenitization degree, and is basically in a tempered state. Even if water cooling is performed during welding, a hardened structure cannot be obtained. Although the width of the heat-affected zone is reduced and its hardness is generally improved, the expected result is not achieved.

[0069] It can be seen that the present invention successfully improves the overall hardness of the DQ steel welded joint significantly.

[0070] The above embodiments are merely examples for clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above descriptions. It is not necessary and impossible to list all the implementation methods here, and the obvious changes or modifications derived therefrom are still within the protection scope of the invention.

Claims

1. A single-side welding hardening process for DQ wear-resistant steel thin plates, characterized in that: The following steps are involved: 1) Parent material properties: DQ wear-resistant steel, carbon equivalent CE is 0.42-0.48%, thickness is 3-8mm, hardness is HV 10 ≥330, -20℃ impact energy KV2≥27J, tensile strength Rm≥1100Mpa; 2) Selection of welding wire and welding process: Estimate the hardness of the weld and heat-affected zone according to the following formula, adjust the welding wire composition and welding process parameters so that the estimated hardness is not less than 87% of the hardness of the parent material; HV=0.75HV1+0.25HV2 HV1=97+949C+27Si+11Mn+8Ni+16Cr+20lgv r HV2=-348+185C+330Si+153Mn+66Ni+144Cr+191Mo+lgv r (89+54C-55Si-22Mn-10Ni-20Cr-33Mo) Where: HV is the estimated hardness, HV1 is the martensite hardness, and HV2 is the bainite hardness; C, Si, Mn, Ni, Cr, Mo are the weight percentages of the elements in the welding wire or base material, in %; the welding wire composition is used to calculate the weld hardness, and the base material composition is used to calculate the heat-affected zone hardness; Vr is the characteristic temperature cooling rate, in K / h; T is the characteristic temperature, T0 is the preheating temperature, q is the welding power, v is the welding speed, and λ is the thermal conductivity of steel; 3) Welding: Use single-sided gas shielded welding. When the overheating zone and normalizing zone of the welding heat affected zone are completely austenitized and the incomplete normalizing zone is partially austenitized, water cooling is performed. Avoid direct contact between water and arc during water cooling. Add water to the welding area without arc. After the arc, the cooling water will cover the welding area by itself and start to cool quickly to form a hardened structure.

2. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The chemical composition and weight percentage of the parent material are: C: 0.15-0.25%, Si: 0.4-0.7%, Mn: 1.2-1.5%, P≤0.02%, S≤0.015%, Cr: 0.1-0.3%, Als: 0.02-0.05%, Ti≤0.03%, and the rest is Fe and unavoidable inclusions.

3. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The chemical composition and weight percentage of the welding wire are: C: 0.06-0.11%, Si: 0.5-0.7%, Mn: 1.5-1.8%, P≤0.02%, S≤0.015%, Ni: 0.8-1.2%, Cr: 0.3-0.5%, Cu: 0.15-0.3%, Ti: 0.06-0.15%, and the rest is Fe and unavoidable inclusions.

4. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The characteristic temperature cooling rate does not exceed the maximum cooling rate of room temperature water quenching of 200°C / s.

5. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The welding current in the welding process is 180-290A, the welding voltage is 20-29V, and the welding speed is 38-70cm / min.

6. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The welding line energy of the welding wire is E=(1-2.5)×t, wherein the unit of E is kJ / cm, and t is the thickness of the base material in mm.

7. The single-side welding hardening process for DQ wear-resistant steel thin plate according to claim 1 is characterized in that: The protective gas for the single-sided gas shielded welding is a mixed gas of 78-82% Ar and 18-22% CO2, and the protective gas flow rate is 10-20 L / min.

8. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The water flow rate of the water cooling is 6-12 L / min, the water temperature is room temperature, the water cooling width is 30-60 mm, and the cooling rate is 75-200° C. / s.

9. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The hardened structure is at least one of martensite and bainite.

10. The single-side welding hardening process for DQ wear-resistant steel thin plates according to claim 1 is characterized in that: The hardened structures formed in the weld, overheating zone and normalizing zone are martensite and bainite, of which the volume proportion of martensite is 60-90%; the hardened structures formed in the incomplete normalizing zone are martensite, ferrite and pearlite, of which the volume proportion of martensite is 30-60%.

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