Gas metal arc welding of high nitrogen stainless steel with Ar-N2-O2 ternary shielding gas

Zeng Liu ,Cheng-lei Fn ,*,Zhu Ming ,Cho Chen ,Ang Liu ,Chun-li Yng ,Sn-o Lin ,Lng-ping Wng

a State Key Laboratory of Advanced Welding and Joining,Harbin Institute of Technology,Harbin,150001,China

b Inner Mongolia Metal Material Research Institute,Baotou,China

Keywords:High nitrogen stainless steel Ar-N2-O2 shielding gas Action mechanism

ABSTRACT High nitrogen stainless steel with nitrogen content of 0.75%was welded by gas metal arc welding with Ar-N2-O2 ternary shielding gas.The effect of the ternary shielding gas on the retention and improvement of nitrogen content in the weld was identified.Surfacing test was conducted first to compare the ability of O2 and CO2 in prompting nitrogen dissolution.The nitrogen content of the surfacing metal with O2 is slightly higher than CO2.And then Ar-N2-O2 shielding gas was applied to weld high nitrogen stainless steel.After using N2-containing shielding gas,the nitrogen content of the weld was improved by 0.1 wt%.As N2 continued to increase,the increment of nitrogen content was not obvious,but the ferrite decreased from the top to the bottom.When the proportion of N2 reached 20%,a full austenitic weld was obtained and the tensile strength was improved by 8.7%.Combined with the results of surfacing test and welding test,it is concluded that the main effect of N2 is to inhibit the escape of nitrogen and suppress the nitrogen diffusion from bottom to the top in the molten pool.

1.Introduction

Nitrogen alloying can improve the strength and corrosion resistance of austenitic steel[1-3].Especially the properties can be enhanced obviously[4]under high strain rate after adding nitrogen to the steel,which can be used to improve the ballistic performance of the material.Therefore,the application of high nitrogen steel(HNS)as the armor material is receiving more and more attention in recent years[5].However,escape and accumulation of nitrogen will happen during the fusion welding process of HNS.This not only increases the ferrite content of the weld[6],but also creates the risk of the defects[7].In addition,the weld that lacks nitrogen will become the weakness when the structure is impacted,which limits the application of the steel in the defence field.As investigated by Hosseini[8],the nitrogen content of the weld has relationship with the arc energy and the initial nitrogen content of base metal.It was highly recommended in their study that the N2-containing shielding gas or filler metal should be used to suppress and compensate the nitrogen loss.Considering that there are no mature high nitrogen welding materials[9,10],it is a very common method to use N2-containing shielding gas[11].Most researches that added N2to the shielding gas mainly aims to increase and stabilize the austenite of the weld[12-15].And the hardness of the solidified zone[12,13]and the corrosion resistance[14,15]of the joint can be thereby enhanced.What’more,it is reported by Elmer[16]that the addition of N2to the shielding gas can also effectively suppress the formation of pores compared to pure Ar.But the N2content is not the more the better,for example,as investigated by Zhao al et.[17],the formation of nitrogen pores is largely depending on the matching of the heat input and the N2proportion.And for different kind of steel,there is always a critical solubility of nitrogen determined by the alloy system and composition content.Exceeding the amount,the increment of nitrogen becomes less obvious and the risk pore formation also increases[18,19].What’s more,as the increasing of N2,the stability and the quality of the weld also decreases,which makes the welding spatter become serious[20].To understand and control the welding process of HNS with N2-containing shielding gas,it is necessary to study the loss and dissolution of nitrogen during the process.The difficulty of the study is that there are many factors affecting the dissolution of nitrogen to the weld,such as the base material[21],filler material[22]and even the area and the holding time of the molten pool[23].Therefore,the researches on the effect of nitrogen have only focused on the increase tendency of nitrogen content and the reduction of ferrite of the HNS weld with N2[24].

In this study,HNS with the nitrogen content of 0.75%was weld by gas metal arc welding with Ar-N2-O2ternary shielding gas.In order to clarify the action mechanism of N2during the welding process,two parts of tests were conducted.The surfacing test is to eliminate the influence of the base material and to select the type of the oxidizing gas.and then the test of HNS butt welding was carried out.The action mechanism of N2was discussed combined two aspects:on one hand,the difference of the nitrogen content between the weld of the surfacing test and the butt-welding test was analyzed;on the other hand,the action process of nitrogen was deduced according to the ferrite content at different positions in the joints with different N2proportion.The test results provide a reference for the optimization and selection of the shielding gas for HNS welding.And a deeper understanding and discussion of the effect of N2can help the promotion and application of N2-containing shielding gas in industrial fields.

2.Experimental procedure

Cr20Mn16NiN high nitrogen stainless steel and Cr20Ni10Mn7Mo were used as the base and the filler material respectively,the chemical compositions of them were given in Table 1.The nitrogen content of the base material is 0.75 wt% and the tensile strength is 1150 MPa Cr20Ni10Mn7Mo is an austenitic stainless-steel consumable with the diameter of 1.2 mm and does not contain nitrogen element.

Surfacing test was conducted to compare the effect of oxidizing gas CO2and O2.The welding power supply was Fronius TPS5000,using KUKA KR16 welding robot to control the welding torch.The welding parameters are given in Table 2.

The welding power of high nitrogen stainless steel welding was Lincoln 500 POWERPLUS.Table 3 shows the welding parameters.

The microscopic morphology of the joints was observed by the GX71 inverted metallographic microscope.As shown in Fig.1,the microstructure of the base material is full austenite.The tensile strength and Vickers microhardness of the joints were tested.Fig.2 describes the dimensions of the base material,welding groove,metallographic sample,microhardness specimen and tensile specimens.The nitrogen content of the weld and heat affected zone(HAZ)was measured by EltraONH-2000 hydrogen-oxygen-nitrogen analyzer.

3.Results

3.1.Surfacing test

Adding a portion of oxidizing gas to the N2-containing shielding gas can effectively improve the nitrogen content of the weld.Assummarized by M.Du Toit[25],formation of NO in the arc,improvement of convergent surface-tension driven flow of the molten pool,occupation of the available surface sites are all the reasons for accelerating the nitrogen dissolution.The formation of surface oxide on the molten pool can also inhibit the nitrogen escape.But the oxidizing gas proportion should not be too high,it will cause the oxide film to be too thick.Surfacing test was conducted according to Table 4 to compare the effect of O2and CO2,the content of O2and CO2is determined to be 2%.

Table 2Welding parameters of surfacing.

Table 3Welding parameters of HNSS welding.

Fig.1.Microstructure of Cr20Mn16NiN steel.

Fig.2.Schematic of the base material and specimens.

Table 1Chemical compositions of the base and filler material.

The nitrogen-free Q235 steel was selected as the base material in order to ensure that the nitrogen of the surfacing metal was all from the shielding gas.15 layers of surfacing were performed,2 beads per layer.As shown in Fig.3(a),the samples were taken from the middle portion of the build-up body to measure the nitrogen and oxygen contents.As shown in Fig.3(b)and(c),the nitrogen content of the samples with N2-containing shielding gas is improved compared with the pure Ar.And the nitrogen content is further improved after the addition of oxidizing gas.In the case of the same proportion of O2and CO2in the shielding gas,the nitrogen content of the samples that using O2as the oxidizing gas is slightly higher than that of the samples added CO2.And the oxygen content of the CO2samples is higher.The results show that the ability of O2to promote nitrogen dissolution to the weld from N2-containing shielding gas is stronger than CO2.O2is thereby selected as the oxidizing gas for the further study.

3.2.Effect of N2 on HNSS welded joints

Use Ar-N2-O2ternary shielding gas to weld 5 mm HNSS plates.The N2content was changed from 10% to 20% to investigate the effect of N2on nitrogen content,microstructure and mechanical properties of HNSS joints.

3.2.1.Nitrogen content

Fig.4 shows the nitrogen content of the weld and HAZ varied with the N2in the shielding gas.There are two main sources of the nitrogen in the weld,namely from the base material and the shielding gas.The nitrogen content of the weld with 2%O2-98%Ar reached 0.40 wt%,which is much higher than the nitrogen content of the surfacing weld without adding N2(0.019 wt%).It means that a considerable part of nitrogen of the base material dissolved into the weld.And after using Ar-N2-O2ternary shielding gas,the nitrogen content has been improved by about 0.1 wt%,but the difference among the welds with 10%N2,15%N2and 20%N2is not obvious.The nitrogen content of the weld with 20%N2increased just a little(0.04 wt%)than that of the weld with 10N2and 15N2.The nitrogen content of the HAZ does not show a significant change with the increasing of N2,which maintained at a level slightly lower than that of the base material.So the N2-containing shielding gas mainly affects the nitrogen content of the weld,while has little effect on the HAZ.

Table 4Shielding gas compositions of surfacing.

Fig.3.Nitrogen content and oxygen content of the surfacing weld with different shielding gas compositions:(a)sampling position(b)nitrogen content(c)oxygen content.

3.2.2.Microstructure

Fig.5 shows the microstructure of the upper,middle and lower parts of the weld.The microstructure of the weld with 98%Ar-2%O2binary shielding gas has no obvious change from top to bottom.They are all skeletal ferrite and austenite matrix.The weld with 10%N2has a similar microstructure to the weld without N2,but the ferrite content was obviously reduced.And when the N2content increased to 15%,ferrite was only found at the bottom of the weld.The full austenitic weld was obtained when the N2content increased to 20%.As shown in Fig.5,the microstructure of full austenite was earmarked by red rectangle.It can be seen that as the N2content of the shielding gas increases,the direction of the ferrite decrease is from the top of the weld to the bottom.

Fig.4.Change of nitrogen content of the joints with different N2 content.

Due to the dissolution of nitrogen from the base material to the weld,the loss of nitrogen in the HAZ near the fusion line results in the formation of large amount of ferrite.Fig.6 shows the microstructure of the fusion zone of the joints with 0%,10%,15%and 20%N2.It can be seen from the figure that the area of the ferrite region and the amount of ferrite of the joints with N2-containing ternary shielding gas are significantly smaller than those of the joint with Ar-O2binary shielding gas.

And as shown in Fig.7,there are no obvious differences in the microstructure of the HAZ on the side of the base material,all of which are austenite.Therefore,the N2-containing shielding gas only has effect on the HAZ near the fusion zone of HNSS joint.

3.2.3.Microhardness profile

Fig.8 shows the microhardness profile of the joints with different shielding gas.The direction of hardness measurements is given in Fig.8(a).The horizontal and vertical direction hardness of the weld are shown in Fig.8(b)and(c),respectively.The microhardness increases from the weld to the base material.The weld hardness is lowest of the joint,which is related to the minimum nitrogen content.And from Fig.8(c),the microhardness of the welds with N2-containing shielding gas is higher than that of the weld without N2.This is because the nitrogen content increased after using N2-containing shielding gas.But the microhardness values of the welds with different content of N2do not show an obvious difference.The sample standard deviation of vertical microhardness was calculated as Eq.(1)to measure the dispersion of the weld hardness.

whereSis the sample standard deviation,Nis the sample account,Xiis the sample value andis the sample mean.

As shown in Fig.8(d),the standard deviation of the weld microhardness elevates significantly after adding N2to shielding gas,but that value decreases with the improving of the N2content.It indicates that the inhomogeneity of the weld increases after using N2-containing shielding gas,but as the N2content increases,the weld will become more and more uniform.The phenomenon is consistent with the observation of the weld microstructure.

3.2.4.Tensile strength

The tensile strength of the joints with different N2content is shown in Fig.9.When the N2proportion reached 20%,the tensile strength was improved from 784 MPa to 852 MPa,which was increased by 8.7%and reached 74%of the base material.The tensile strength of the joint is improved obviously after using N2-containing shielding gas.And although the difference of the nitrogen content in the weld with 10%,15%and 20%N2is not so much,there is still an increase in the tensile strength.This is related to the ferrite reduction and the uniformity of the weld.

Fig.10 shows the tensile fracture morphology.The dominant fracture behaviors were all ductile fractures.And the dimensions of the dimples of the weld with N2-containing shielding gas are markedly smaller than the weld without N2.

Fig.5.Microstructure of the welds with different N2 content.

4.Discussion

In the surfacing test,the application of Ar-N2-oxidizing gas to add nitrogen achieved a certain effect.The nitrogen content of the weld was maintained between 0.2 wt% and 0.3 wt%,and it is noteworthy that the nitrogen content of the weld with 10% N2is generally higher than that with 15% N2as shown in Fig.3,which means that the effect of nitrogen addition by shielding gas is unstable.Because it is limited by the compositions of the weld and the stability of the welding process.When welding the HNS plates,the nitrogen content of the HNS weld without N2reached 0.4 wt%,which is much higher than the surfacing metal without N2(0.019 wt%).After using the N2-containing shielding gas,the weld nitrogen content has been increased by about 0.1 wt%,but as the nitrogen content continued to increase,the nitrogen content of the weld did not change obviously.Because of the application of the nitrogen-free filler metal,there are two main sources of nitrogen in the HNS weld:the shielding gas and the base material.And in the surfacing test,the influence factor of the base material was excluded.Therefore,the nitrogen dissolved from the base material accounts for the vast majority of the nitrogen content in the weld.It should be stated that the equipment of the surfacing test and the welding test is different.But the two parts of the tests are relatively independent and the heat input of the two welding processes is close,so the results can be used for qualitative comparative analysis.

Fig.6.Microstructure of the fusion zone with different proportion of N2:(a)(b)0%(c)(d)10%(e)(f)15%(g)(h)20%.

Therefore,it is the compositions of the material determining the nitrogen content of the weld,instead of the N2pressure of the shielding gas.As investigated by Park et al.[19],the critical amount of the nitrogen content is directly affected by the alloy compositions of the steel,but has little to do with the size of the material and the cooling options.So the shielding gas compositions do not play a decisive role in the nitrogen content of the material,but it can be seen as an assistant means to optimize the microstructure and add nitrogen to the molten pool during the welding process.As investigated by Zhao al et.[26],nitrogen addition can hinder the phase interface migration during the ferrite→austenite transformation and accelerate the nucleation and growth of austenite.The distribution of austenite and ferrite can reflect the nitrogen distribution in the weld.As shown in Fig.5,the volume fraction of the ferrite decreases from top to the bottom of the weld with the increasing of N2,which leads to the uniformity of the weld,such as microstructure and microhardness,is improved.It is considered that it is the key to improve the joint performance.Besides,there is also a part of nitrogen diffusing from the HAZ to the weld and forming a nitrogen lost region,which is reflected by the appearance of ferrite in that region.As shown in Fig.6,the volume fraction of ferrite in HAZ near the fusion line was reduced obviously with the using of N2-containing shielding gas.It indicates that the nitrogen-containing shielding gas can effectively suppress the escape of nitrogen in HAZ.

Fig.7.Microstructure of HAZ with different proportion of N2:(a)0%(b)10%(c)15%(d)20%.

Fig.8.Microhardness profile of the joints with different shielding gas:(a)diagram of microhardness direction(b)horizontal direction(c)vertical direction(d)standard deviation of vertical microhardness.

Fig.9.Stress-displacement curves of the joints with different N2 content.

Combined with the measurement results of the nitrogen content in surfacing and HNSS weld and the evolution of the joint microstructure with N2increasing,the action mechanism of N2is analyzed.As shown in Fig.11,when using pure Ar to weld HNSS,the nitrogen of the weld is all derived from the base material and the nitrogen of the molten pool escapes from the lower to the upper.With the application of N2-containing shielding gas,the direction of nitrogen diffusion is from the surface of the molten pool to the interior.It is proved by the test results studied by Dong et al.[27],the nitrogen content of the lower part in the weld by laser with N2-containing shielding gas is less than the upper part and their nitrogen content are all greater than the nitrogen-free base material.A modified Fick’s law equation can be used to describe the nitrogen diffusion on the surface of molten pool.[28],as shown in Eq.(2).

where dmis the net nitrogen content,Apwis the weld top area,Rpwis the mass transfer coefficient between plasma and weld,apis the nitrogen activity of the plasma,a0wis the nitrogen activity of the weld just under the plasma,Awgis the weld top area,Rwgis the mass transfer coefficient between weld and shielding gas,awis the nitrogen activity of the weld,agis the nitrogen activity of shielding gas.

The nitrogen activity of the plasma(ap)and shielding gas(ag)increase with the N2proportion of the shielding gas.This is why the nitrogen content of the weld increase obviously with N2-containing shielding gas compared with the weld with pure Ar.But as the N2proportion continue to increase,the nitrogen activity of the weld(awanda0w)will be also increased.The dissolution rate of nitrogen is decreased.The increment of nitrogen is thereby not obvious.At the same time,the increase of N2pressure in the arc space also leads to the loss rate of nitrogen in the molten pool reduced.Nitrogen element accumulates on the top of the molten pool.Due to the strong ability of nitrogen to form austenite,the ferrite in the upper part of the weld decreases to disappear.As the N2pressure continues to increase,the nitrogen diffusion rate of the molten pool surface is further reduced,resulting in a decrease in the diffusion of nitrogen from the lower part to the upper of the molten pool,so the nitrogen distribution is more uniform,which is reflected by the full austenitic weld was obtained when the N2content of the shielding gas reached a certain value.In addition,the decrease of the nitrogen diffusion rate is also manifested at the solid-liquid interface of the fusion line,which results in the area reduction of the nitrogen lost region of the HAZ.

5.Conclusions

In this work,the action mechanism of N2was discussed by the nitrogen content comparison of the surfacing test and HNSS welding test and the analysis of the microstructure evolution of the joints with different proportion of N2in shielding gas.It is concluded that the shielding gas has no decisive effect on the nitrogen content of the weld.The main function of N2-containing shielding gas in the process of fusion welding is to suppress the escape of nitrogen to make the weld uniform.The detailed conclusions are as follows:

Fig.10.Tensile fraction morphology:(a)0% N2(b)10% N2(c)15% N2(d)20% N2.

Fig.11.Schematic of the action mechanism of N2.

1.Both for the surfacing test and the welding test,the application of the N2-containing shielding gas can obviously increase the nitrogen content of the weld.But the change of the nitrogen content was not significant with the continued increase of the proportion of N2.

2.With N2content increasing in the ternary shielding gas,the ferrite of the weld reduced from the top to the bottom and the nitrogen-lost region of the HAZ decreased.

3.The main reason why adding N2can improve the joint performance is that N2-containing shielding gas inhibits the escape of nitrogen in the molten pool so that the austenite content increases and the uniformity of the weld microstructure is improved.