Effect of the heat input on microstructure and properties of submerged arc welded joint of 08Cr19MnNi3Cu2N stainless steel

Dai Hong,Xia Xiwei,Fang Naiwen,Ma Qingjun,Chang Jingshu

1.Harbin Welding Institute Limited Company,Harbin 150028,China;

2.China Classification Society Guangzhou Branch,Guangzhou 511458,China;

3.Tianjin Special Equipment Institute,Tianjin 300192,China

Abstract SAW308L submerged arc welding wire and SJ601A submerged arc welding flux were selected to weld the 12 mm 08Cr19MnNi3Cu2N low nickel and high nitrogen austenitic stainless steel plates with three different welding heat input,and microstructure,tensile properties,microhardness and corrosion properties of the welded joints were studied.The results show that no defects are found in the three groups of welded joints,and the welded joints have better performance.The tensile strength of 08Cr19MnNi3Cu2N stainless steel welded joints with different heat input is slightly lower than that of the base metal,and fracture occurs in the weld zone,and the hardness of the weld zone is lower than that of the base metal.The weld microstructure of stainless steel welded joints with different heat input is composed of austenite+δ ferrite,and ferrite is uniformly distributed in austenite.With the increase of the welding heat input,the ferrite content in the weld zone decrease gradually,the grain size in the thermal affected zone increase gradually,and the impact toughness reduce.

Key words heat input,submerged arc welding,low nickel and high nitrogen austenitic stainless steel,microstructure,property

0 Introduction

Stainless steel has the characteristics of superior comprehensive mechanical properties and good corrosion resistance,and is the key development object in the next generation vehicle program (NGVP)[1].Due to its considerable saving of nickel resources,low nickel and high nitrogen austenitic stainless steel has become a focus of research and application in recent years[2].It use N element instead of expensive Ni element as the main austenitizing element.Due to the high stacking fault energy of nitrogen element in steel[3],it not only optimizes the material microstructure,but also greatly improves the comprehensive properties of the material.The low production cost,excellent corrosion resistance and biological compatibility of low nickel and high nitrogen austenitic stainless steel make it have wide application prospect in shipbuilding,aviation weapon equipment fields and so on[4].However,the solid nitrogen is easy to escape in the welding process,which makes the welded joint performance of low nickel and high nitrogen stainless steel decrease,and limits its popularization and application,especially in the field of large heat input submerged arc welding technology of medium and thick plate,there are few researches were carried out at present.

Therefore,the most direct way to solve this problem is to select reasonable welding heat input to control the nitrogen content in the weld pool and ensure the quality of the welded joint[5].Taking 08Cr19MnNi3Cu2N low nitrogen and high nickel austenitic stainless steel plate for research,the microstructure and properties of welded joint are studied in detail by MAG welding process.The results show that the steel has good welding technology forming,good mechanical properties of welded joints,and is suitable for welding with high heat input conditions,which provides a theory basis for its application in actual engineering.

1 Materials and methods

1.1 Materials

The thickness of 12 mm 08Cr19MnNi3Cu2N low nickel and high nitrogen austenitic stainless steel plate produced by Fujian Qingtuo Special Steel Technology Research Co.,Ltd.was selected as the base metal.The diameter of 3.2 mm SAW308L submerged arc welding wire and the 10-60 mesh SJ601A submerged arc welding flux produced by Harbin Well Welding Co.,Ltd.were selected as the welding materials,and its slag system was CaF2-CaO-Al2O3-SiO2,which was alkaline.The chemical compositions of stainless steel plate and submerged arc welding wire are shown in Table 1,and the mechanical properties are presented in Table 2.

1.2 Methods

The test adopted submerged arc welding technology.The size of butt welding test plate was 300 mm×150 mm×12 mm.The groove angle of butt welding test plate was 60 °,the truncated edge was 2 mm,the gap of test plate was 1.5 mm,and about 10° of inverse distortion was reserved.Fig.1 is schematic diagram of groove size.Specific welding parameters are shown in Table 3.

Non-destructive testing was carried out according to GB/T 3323—2005 Radiographic examination of fusion welded joints in metallic materials and GB/T 26953—2011 Non-destructive testing of welds —Penetrant testing of welds—Acceptance levels after the completion of the test plate welding.The sample size,sampling methods and test methods respectively should be carried out according to GB/T 25774.2-2016 Test methods for welding consumables—Part 2:Preparation of single-run and two-run technique test specimens in steel,GB/T 26955—2011 Destructive tests on welds in metallic materials—Macroscopic and microscopic examination of welds,GB/T 2651 —2008 Tensile test method on welded joints,GB/T 2650—2008 Impact test methods on welded joints,GB/T 2653—2008 Bend test methods on welded joints,GB/T 2654—2008 Hardness test methods on welded joints,GB/T 4334—2008 Corrosion of metals and alloys—Test methods for intergranular corrosion of stainless steels and relevant technical requirements.

Table 2 Mechanical properties of the base matal and welding material

Fig.1 Schematic diagram of groove size (mm)

2 Results and discussion

2.1 Macro-profile and inspection results of welded joints

Fig.2 shows the macro-profile of welded joints with different heat input.The joints are well combined without obvious welding defects such as pores,cracks,incomplete fusion and slag inclusion,etc.

The results of radiographic inspection and penetrant inspection of the welded joint with the three groups of heat input are shown in Table 4.The welded joints have no slag inclusion,incomplete fusion,undercut and other defects,and had only a few pores.When the welding heat input increase from 1.7 kJ/mm to 2.1 kJ/mm,a few pores basically disappear.This is because that increasing of welding heat input,the volume of molten pool increases and the existence time is longer,so that the pores in the molten pool have sufficient escape time.Moreover,the increase of welding heat input can aggravate the stirring effect of arc on the molten pool,promote the escape of pores,and make pores disappear[6].

Table 3 Specific welding process parameters

Fig.2 Macro-profile of welded joints with different heat input (a)2.1 kJ/mm (b)1.9 kJ/mm (c)1.7 kJ/mm

2.2 Mechanical properties of welded joints

Table 4 Radiographic and penetrant inspection results

2.2.1 Tensile and bending test results

The tensile and bending test results of the welded joints are shown in Table 5.The results show that the tensile properties are all slightly lower than the base metal and broken in the weld zone.Surface bending and back bending tests were carried out on the welded joints at room temperature,and macroscopic observation was made on the welded joints.No cracks,no holes or other defects were observed.It indicates that the welded joints have sufficient plasticity.

2.2.2 Impulse test results

The impulse test results of the welded joints are shown in Table 6.The results show that 08Cr19MnNi3Cu2N austenitic stainless steel welded joints have good low temperature impact resistance.Under the action of welding heat circulation,the solid solution nitrogen in the base metal has different precipitation tendency,which is closely related tothe welding heat input[7].With the increase of welding heat input,the precipitation of nitride is intensified,which reduces the impact toughness of the welded joint and the content of solid solution nitrogen in the matrix,leads to the stability of austenite get worse.During welding process,the welding heat input and the temperature between weld bead in multi-pass welding are strictly controlled to reduce the high-temperature residence time so as to reduce the nitride precipitation in the heat affected zone and the front weld zone.

Table 5 The tensile and bending test results

Table 6 Impact absorbed energy at different temperatures (J)

2.2.3 Hardness test results

The microhardness test results of the welded joints are shown in Table 7.The results show that the hardness of the welded joint in the heat affected zone increases slightly,but the other positions have no obvious hardening tendency.The microhardness of the weld under the three groups of heat input is lower than that of the base metal.This is mainly caused by the overflow of nitrogen from the weld zone in the welding process.The smaller the grain size,the more grain boundaries,the greater the resistance to dislocation movement,and the greater the resistance to material deformation[8].Macroscopically,the hardness is high,so the overall hardness distribution of welded joints decreases gradually with the increase of heat input.In addition,with increase of the heat input,the hard brittle carbide precipitated from the welded joint gradually decreases until it disappears,so its hardness gradually decreases.

As can be seen from the microstructure analysis,with the increase of welding heat input,the austenite content in the weld microstructure gradually increases,and the hardness of austenite is lower than that of ferrite,so the hardness of the weld zone also decreases.Among the three regions,the hardness of the heat affected zone is the highest.It is because of the metal elements in the heat affected zone burned in high temperature,which has little influence on the microstructure.The grain size is not significantly larger than that of the base metal,and the brittle carbide is precipitated out in the heat affected zone,so the hardness of the heat affected zone is higher.

Therefore,there is no obvious softening zone in low nickel and high nitrogen austenitic stainless steel under suitable welding heat input.

Table 7 Microhardness test results (HV5)

2.3 Microstructure

Fig.3 is the microstructure morphology of the welded joint with the heat input of 2.1 kJ/mm.The microstructure morphology of the weld zone is shown in Fig.3a,and the microstructure is austenite andδferrite.The γ solid solution content in 5 randomly selected fields is about 90.1%.The microstructure of the overheated zone is austenite+δferrite,as shown in Fig.3c,and the grain size number is 7.5.

Fig.4 is the microstructure morphology of the welded joint with the heat input of 1.9 kJ/mm.The microstructure morphology of the weld zone is shown in Fig.4a,and the microstructure is austenite andδferrite.The γ solid solution content in 5 randomly selected fields is about 89.9%.The microstructure of the overheated zone as shown in Fig.4c is austenite+δferrite,and the grain size number is 7.5.

Fig.5 is the microstructure morphology of the welded joint with the heat input of 1.7 kJ/mm.The microstructure morphology of the weld zone is shown in Fig.5a,and the microstructure is austenite andδferrite.The γ solid solution content in 5 randomly selected fields is about 89.7%.The microstructure of the overheated zone as shown in Fig.5c is austenite+δferrite,and the grain size is about 7.5.

Fig.3 Microstructure morphology of the welded joint with the heat input of 2.1 kJ/mm (a)The weld zone (b)The fusion zone (c)The overheated zone

Fig.4 Microstructure morphology of the welded joint with the heat input of 1.9 kJ/mm (a)The weld zone (b)The fusion zone (c)The overheated zone

Fig.5 Microstructure morphology of the welded joint with the heat input of 1.7 kJ/mm (a)The weld zone (b)The fusion zone (c)The overheated zone

The welding heat input has a great influence on the cooling speed and the residence time at the high temperature of the weld pool.It changes the G/R(G/R is the ratio of the temperature gradient in front of solidification and solidification rate.)value in the solidification process,so that different microstructure distributions can be obtained in the weld zone[9].The weld microstructure of the three groups of samples is composed of austenite+δferrite,and ferrite is uniformly distributed in the austenite.The dendrite morphology is mainly skeleton and slab.According to the change of γ-solid solution content in the weld zone under the condition of three different welding heat input,with the increase of heat input,the cooling speed of weld pool slows down,and the time for the microstructure to pass through the twophase zone increases,δ→γ reaction process can be fully carried out,leading to the residual γ content gradually increase,which can also be verified from the XRD quantitative analysis in Fig.6.There is no obvious fusion line in the fusion zone of the three groups of samples,and the continuous transition within the grain indicates that the base metal and weld zone are well fused.The coarse grain of the overheat area is generally caused by the heating,so this part also becomes the weakest part of the welded joint.Microstructure in the overheat zone of heat input of the three groups is not produced the phenomenon of grain growth,which means that the performance of the welded joint does not reduce[10].

Fig.6 XRD diffraction analysis

It may be that the solid solution strengthening effect of a large amount of nitrogen in the welded joint and the "nailing" effect of intercrystal precipitation of Cr2N effectively hinder the movement of grain boundary during the short time of welding heating,and inhibits the grain to grow and merge.

2.4 Intergranular corrosion test

After cleaning,the three groups of samples are put into a beaker containing the corrosive solution of sulfuric acidcopper sulfate and copper dust prepared according to GB/T 4334—2008 E method,and are boiled for 16 hours continuously.The specimens are then bent to 180° using a 5 mm rotary head.The bending surface of the samples is observed,as shown in Fig.7,no intergranular corrosion tendency is found.

Fig.7 Intergranular corrosion samples

3 Conclusion

(1)The welded joints of different heat input are well combined without obvious welding defects such as pores,cracks,incomplete fusion and slag inclusion.The results of radiographic inspection and penetrant inspection of the welded joint show that the welded joints have no slag inclusion,incomplete fusion,undercut and other defects,only have a few pores.

(2)The tensile strength of 08Cr19MnNi3Cu2N austenitic stainless steel welded joints with different heat input is slightly lower than that of the base metal,and fracture occurs in the weld zone,and the hardness of the weld zone is lower than that of the base metal.The impact toughness decreases with the increase of welding heat input because of nitrides precipitation increasing in the weld zone and nitrogen solid solution decreasing in the base metal.

(3)The weld microstructure of 08Cr19MnNi3Cu2N austenitic stainless steel welded joints with different heat input is composed of austenite+δferrite,and ferrite is uniformly distributed in austenite,the dendrite morphology is mainly skeleton and slab.The overheat area is the weakest part of welded joint because of the coarse grain caused by heating.