Study on microstructure characterization and mechanical properties of AISI 444 Ferritic Stainless Steel Joint by high frequency pulse K-TIG welding

: In this study, high frequency pulse K-TIG was used to weld AISI 444 ferritic stainless steel (FSS) with a thickness of 6 mm, and the welding process window was determined, and complete penetration was achieved in the state of single pass autogenous welding. The influence law of technological parameters and weld appearance was analyzed. The microstructure characteristics of weld zone (WZ) and heat-affected zone (HAZ) was analyzed. Finally, the mechanical properties of welded joints are evaluated. It is found that the thermal conductivity of FSS is large, so the welding process window is relatively narrow, and the welding heat input determines whether the weld can achieve complete penetration. The second pha se, such as σ phase, χ phase and Laves phase, is precipitated in WZ and HAZ phases. At the crystal boundary, the σ phase and χ phase appears, while the Laves phase is dispersed in the intragranular and crystal boundary. In addition, NbC is found in the HAZ. The brittle-hard phase and coarse grain have great influence on tensile properties and impact toughness at room temperature. The tensil e strength of the weld is equivalent to 70.57% of the parent material, which is 596 MPa. Its average impact toughness value is 29.7J, which is equivalent to 43.04% of the BM. Although the strength and toughness of FSS joints welded with high-frequency pulse K-TIG are lower than that of BM, the welding efficiency is significantly improved, and it is suitable for components with slightly lower load requirements.


Introduction
Ferritic Stainless Steel(FSS) contains almost no Ni and is a cheaper alternative to austenitic stainless steel (ASS).FSS has the advantages of high thermal conductivity, low linear expansion coefficient and good stress corrosion resistance.At present, FSS has been widely used in pressure vessels, automotive industry [1][2][3] and other fields.
However, the grain coarsening of the weld zone (WZ) in the welding process of FSS will seriously reduce the toughness, ductility and corrosion resistance of the welded joint [4][5][6].
It is usually possible to use filled austenitic welding wire to solve the problem of reduced toughness of the weld, such as SMAW [7], GMAW [8,9], FCAW [10], etc.However, this will increase the welding material loss, reduce the welding efficiency, and increase the welding cost.TIG welding is an ideal welding method for FSS.This is because of the stable arc, high welding quality and low welding heat input of TIG welding.However, due to the low arc energy of TIG welding, it is not suitable for the welding of medium and thick plates, so only multi-pass gas tungsten arc welding can be used, which seriously affects the welding efficiency [11][12].
Deep penetration welding techniques such as electron beam welding (EBW) [13,14], laser beam welding (LBW) [5,15] and plasma welding (PAW) [16] can improve the efficiency of medium and thick plate welding.However, these welding methods are not suitable for field operation, the equipment is complex, and the production cost is high, and the practical application is limited [17].K-TIG is a kind of keyhole deep penetration welding method derived from traditional TIG welding.Different from traditional TIG, in order to carry greater welding current (>300A), K-TIG welding gun uses a larger diameter tungsten electrode, welding gun can be applied to strengthen the tungsten electrode cooling system.High current causes arc self-compression, which, combined with efficient cooling system, results in arc compression effect [17].The arc energy and arc pressure are increased after compression.
The larger arc pressure causes the liquid metal in the molten pool to flow around and pit until it penetrates.Under the combined action of arc pressure, the gravity of the liquid metal, and the surface tension of the liquid metal, the small hole remains in dynamic equilibrium [18,19].During the welding process, the keyhole is kept open, and part of the welding arc spills over the back of the workpiece, as shown in Fig. 1(a).It has been proved that K-TIG can achieve full penetration of medium and thick plates in the state of autogenous welding [17].Compared with LBW, EBW and PAW, K-TIG has easy operation, low cost and low pre-welding assembly requirements, which is suitable for field work [18,20].
It has been proved that K-TIG has been successfully used for single pass welding of medium and thick plates of Zr [21], Ti [22] and other materials.Fei et al. [23] used K-TIG to achieve complete penetration of armour grade quenched and tempered steel in the state of autogenous welding.The weld is composed of dendrite and bainite structure, the hardness of weld is increased, and the ballistic performance is improved.In recent years, scholars have conducted a lot of research on the K-TIG welding process of stainless steel.
Feng et al. [20] adopted K-TIG to weld 316L ASS with a thickness of 6-13mm, welded with I-groove without filling welding wire, and completed one-time welding.The tensile strength and impact toughness after welding are similar to that of the base metal (BM).K-TIG is a high quality and efficient welding method for AISI 316L ASS.Cui et al. [24] welded 8mm thick 2205 duplex stainless steel with K-TIG at a welding speed of 280-340mm/min, achieving one-side welding with back formation forming of 2205 DSS welds with good joint plasticity after welding.Zmitrowicz et al. [25] verified the applicability of K-TIG welding for one-side welding with back formation of 1.4462 duplex steel.The results show that the ferrite content of K-TIG weld zone is increased, but the microstructure of heat-affected zone of K-TIG welding is similar to that of traditional TIG welding, and the properties of welded joint meet the requirements.Cui et al. [26] studied the K-TIG welding of 304 ASS with a thickness of 4mm, and realized the "One pulse one open keyhole" K-TIG welding process under the square wave pulse welding current.Under the condition of low average current, a qualified welded joint is obtained.
At present, the research on K-TIG stainless steel welding mainly focuses on ASS and DSS, while the research on FSS with higher thermal conductivity is relatively rare, so exploring the high-frequency pulse K-TIG welding of FSS has additional research value.
In this study, high frequency pulse K-TIG welding was used to improve the possibility of weld quality of AISI 444 FSS.The process window of K-TIG welding of AISI 444 FSS was investigated.The microstructure characterization and mechanical properties of welded joints with different parameters were studied.New welding methods and research ideas are provided to solve the problem of high efficiency and high quality welding of medium and heavy plate FSS.

Materials and experiment methods
AISI 444 FSS is used as the BM in this experiment, and its chemical composition is shown in Table 1.The test plate size is 150 mm×80 mm×6 mm.The high-frequency pulse K-TIG welding system is shown in Fig. 1(a).The welding power supply adopts self-developed high-frequency pulse K-TIG welding, and the welding current adjustment range is 10~1000 A. Two Charpy V impact toughness samples with dimensions of 55 mm × 10 mm × 2.5 mm were taken from the weld zone, and the impact test was carried out at room temperature.

Study on High frequency pulse K-TIG welding process window
Because the arc pressure, surface tension and gravity of liquid metal in the molten pool need to be dynamically balanced when K-TIG welding forms the keyhole, in order to meet the stable advance of the keyhole, the welding parameters need to be strictly equal, such as: tungsten electrode diameter, tungsten electrode spacing, tungsten tip angle, welding current, welding speed and argon flow rate, so the welding process window is very narrow, and the welding process window should be explored.After a large number of process parameter exploration experiments and literature review, in the case of other welding parameters constant, the choice of welding speed in 4.5mm /s~6.5mm/s.In this experiment, a cerium tungsten electrode with a diameter of 6 mm and a tungsten tip Angle of 90° was used.FSS has high thermal conductivity, welding arc ability is easy to be dispersed, reduce the distance between the tungsten pole and the workpiece helps to reduce the loss of energy, and the keyhole has a certain constraint on the arc, after repeated experiments, choose the tungsten pole spacing of 0.5mm.In this experiment, argon gas with purity of 99.999% was used for protection, and the gas flow rate was 25 L/min.Too large or too small gas flow rate will lead to air involvement and affect the welding quality.The welding process parameters of each group in the high-frequency pulse K-TIG welding test are shown in Table 2. Fig. 2 shows the weld appearance of AISI 444 FSS welding with high-frequency pulse K-TIG under different welding parameters.Single-pass welding was completed in all welding parameters, most of the welding appearance was of good quality, and the overall weld appearance color was composed of black and silver white, which was due to too large welding current and insufficient gas protection.Because the welding current is very large, the arc quenching pit is formed at the end of the weld.In Fig. 2 (g), the tail of the weld is burnt through, the middle of the weld is convex, and the quality of the weld is poor, which is caused by excessive welding heat input, and the smaller welding speed will increase the heat input.According to the test results under different welding parameters, the AISI 444 FSS high-frequency pulse K-TIG welding process window is drawn, as shown in Fig. 2. The combination of welding parameters in the blue area can result in a qualified weld.
Qualified welds shall be thoroughly penetrated without weld collapse.Beyond the blue area, prone to weld collapse or incomplete welding phenomenon.Literature [17] points out that welding process window is mainly determined by welding heat input.Too large welding current or too slow welding speed will cause the welding heat input to increase, the small hole dynamic balance is easy to break, resulting in weld collapse.Too small welding current or too fast welding speed will lead to smaller welding heat input, unable to form small holes, resulting in incomplete welding.This can be verified by the cross section morphology of AISI 444 FSS high frequency pulse K-TIG weld under different welding parameters.
Where Q is the welding heat input, J/cm; U is the arc voltage, V; I is the welding current, A; v is the welding speed, cm/s;  is the power coefficient of arc.
Fig. 4 shows the influence of welding parameters on the appearance of the weld.No. 2, No. 3 and No. 8 are the weld appearance when the welding current is 620A and the welding speed rises from 4.5mm/s to 6.5mm/s, respectively.With the increase of speed, the weld penetration becomes smaller and the depth to width ratio of the weld decreases.
Since the welding speed is inversely proportional to the welding heat input, when the welding speed is small, the energy transferred by the arc to the weld increases, which helps to melt the weld.Because the heat in the weld has enough time to be transmitted around, more metal is melted, so the weld weld width is larger, the weld is completely melted, a large amount of metal will flow under the hole, and the excess weld metal of the weld back will increase.When the welding speed is increased, the penetration ability of the arc decreases, and the weld pool is not easy to form a small hole, and the penetration cannot be completed.No. 3 to No. 7 (Fig. 4(c)~(g)) are the high frequency pulse K-TIG weld morphology when the welding speed is 5.5mm/s and the welding speed increases from 620A to 660A, respectively.As shown in Fig. 5, with the increase of welding current, the weld penetration becomes larger, and the depth to width ratio of the weld increases.This is because arc penetration increases with increasing welding current, a similar phenomenon has been found by Feng [20] and Unnikrishnan [27] in ASS welding.When the welding current is 620A~640A, the welding current is small, the penetration of the welding arc is insufficient, the plasma can not be ejected from the back of the weld, can not form a small hole, and the weld has no penetration.The welding current is increased to 650A, and as the welding current increases, the welding arc pressure also increases, and a small hole is formed in the weld pool to realize the one-side welding with back formation.The welding current is further increased to 660A, because the welding current is too large, the weld surface appears concave.Fei et al. [23] found that excessive heat input and arc force would destroy the force balance in the small hole channel, causing the weld to collapse in some areas.In summary, the welding heat input determines whether the high frequency pulse K-TIG weld can complete penetration, the welding current plays a major role in the size of the weld penetration, and the welding speed mainly affects the weld width.Therefore, in the range of good welding process window should match the appropriate welding parameters to obtain the ideal weld forming quality.substituting the chemical composition content in Table 2.1 into the formula, it can be seen that A=20.43,B=0.365.It can be seen from the Schaeffler microstructure chart that the weld structure of AISI444 FSS is a single ferrite structure [20].
The Schaeffler microstructure diagram only considers the effect of chemical composition on the microstructure, but when the alloying element morphology, welding method and joint form are changed, the weld microstructure of AISI444 FSS cannot be predicted more accurately.For example, when the alloying element is precipitated as a compound, it does not affect the phase ratio.Therefore, in order to improve the accuracy of weld microstructure prediction, JMatPro 7.0 material property simulation software was used to simulate the equilibrium phase diagram of AISI444 FSS at 1500℃.
As can be seen from Fig. 6, when the liquid phase cooling of AISI444 FSS is above 800℃, the weld structure is a single ferritic structure.Almost no carbon, nitride precipitation.When cooled to 800℃~500℃,  phase may appear in the weld tissue.
The  phase is Fe-Cr intermetallic compound, which helps to improve the weld strength, but will lead to a serious decrease in plastic toughness.When the cooling temperature is below 500℃, a mixture of '  and Laves phases may appear in the weld tissue.The '  phase is an intermetallic compound rich in Cr.Silva et al. [7] used the time-temperature precipitation (TTP) diagram of FSS (Fig. 7) to predict the second phase precipitation in the weld of FSS.The study found that although Nb in the weld usually exists in the form of carbide or nitride, when the temperature reaches more than 1100℃ (WZ temperature is greater than 1100℃), carbon and nitrogen will redissolve, and Nb will combine with iron and chromium and other elements to produce Laves phase.Zhang et al. [9] pointed out that FSS is sensitive to intergranular corrosion.In order to reduce the phenomenon of intergranular Cr poverty, some stable elements, such as Nb or Ti, are usually added to promote the formation of carbides of these elements, thereby reducing the formation of chromium carbide and preventing intergranular Cr poverty.Since the content of C and N of the material used in this test is very low, only niobium is added to the material, and the method of single stabilization is adopted.
Therefore, carbides and nitrides of Nb may be precipitated in the BM or near the HAZ.
Since the weld will naturally cool to room temperature after welding, the cooling rate is  As the welding heat input of high-frequency pulse K-TIG is large, the grains of the HAZ are coarsed.As can be seen from Fig. 8 (a), the HAZ at the upper part of the weld is very narrow, and the grain size changes little.On the one hand, this is due to the large thermal conductivity of FSS, and on the other hand, it is due to the more concentrated energy density of K-TIG arc, which shorens the high temperature residence time of HAZ.Relatively speaking, the HAZ at the lower part of the weld has larger grain size.This is because the formation of the keyhole helps to deposit arc energy to the bottom of the keyhole [29], so the heat input at the bottom of the keyhole is greater.In addition, the cooling rate of the HAZ is relatively fast, so the HAZ range is small.Corrosion pits after corrosion were found in the HAZ, as shown in Fig. 8 (f).It is pointed out above that the weld tissue may contain some second phase, so further magnification is performed for observation.The presence of some precipitated phases in the grain boundaries and intracrystalline of WZ and HAZ was observed.Fig. 9 (a) shows the microstructure of WZ of FSS.It was observed that precipitates were precipitated at the grain boundaries.EDS was used for quantitative analysis of the precipitates, and it was considered that the grain boundary precipitates were  phase.This is because the  phase is generally precipitated at the grain boundary of ferrite, and is metal Fe-Cr phase, so it is a Cr-rich phase, and when the  phase grows to a certain extent, it will precipitate from the inside of the grain.Sourmail et al. [7,30] found that  phase is easy to form when the mass fraction of Cr is 25%~30% and the precipitation temperature is 600℃~1050℃, and Cr, Nb, Mo, and other elements can promote the formation of  phase.FSS contains the above elements, the enrichment of elements in the weld and the temperature range of the weld provide favorable conditions for the formation of  phase.According to Fig. 9 (a) and Fig. 9 (b), a very small precipitate was precipitated at the grain boundary, which was identified as the  phase.Lu et al. [31] found that  phase is generally precipitated at the grain boundary, which is characterized by nanoscale rod shape.Ge et al. [32,33] believe that a certain amount of Mo element in FSS will promote the precipitation of  phase.Generally, the  phase is easy to form at a Mo mass fraction of 15% to 25% and precipitation temperature of 600℃ to 900℃.Research has found that  precipitates faster and earlier than  phase [32,34], and  phase is considered as a promoter of  phase [7].This is because  coexists in a metastable state and gradually decomposes with time and temperature, providing a large amount of Mo and Cr elements for the formation of  phase.A large amount of needle precipitate was also found in the WZ of the high-frequency pulse K-TIG connector, as shown in Fig. 10.According to the results of EDS analysis (Fig. 10 (a)), the precipitated phase was mainly composed of Cr, Fe, Nb and other elements, and the sediment was mainly distributed in the crystal, with a small amount distributed in the grain boundary, so it was judged that the sediment was Laves phase.
Ma et al. [35] pointed out that compared with  phase and  phase, the most significant feature of Laves phase is the higher content of Nb element.Moreover, the mass fraction of Nb and Mo in Laves is higher than that in the matrix, and the mass fraction of Nb and Mo in Laves is generally precipitated at the position of dislocation and substructure.Fig. 10 (c)~(i) shows the EDS map scanning results in WZ.It can be seen that there is element segregation in the sediments, and Nb element has obvious segregation.The results confirm that the Laves phase in the crystals may be Fe 2 Nb.In addition, as shown in Fig. 10 (b), in addition to acicular precipitates, spherical precipitates were also found inside the grains in the WZ region.EDS analysis results showed that the content of Mo element in spherical precipitates was relatively high.
Juuti et al. [36] believed that the Laves phase in FSS might be Fe 2 Nb and Fe 2 Mo, and Si element in FSS would be enriched to precipitates after precipitation.Therefore, it is speculated that the sediment is also Fe 2 Mo type Laves phase.Both  and  phases and Laves phases will reduce the toughness and increase the hardness of FSS.
According to Fig. 11 (a), it can be found that in addition to the acicular Laves phase, the WZ region also contains massive precipitated phases, which may be Cr-rich phase '  according to the analysis above (Fig. 6).It has been pointed out that when the mass fraction of Cr in FSS is greater than 12%, it will produce 475℃ embrittlement under the condition of long-term insulation at 340℃~516℃, and it is generally believed that at this temperature, ferrite will decompose into Cr-rich '  phase and Fe-rich  phase with the same bcc structure.The mass fraction of Cr in '  phase ranges from 61% to 83%, while the mass fraction of Fe in EDS in Fig. 11 (b) is larger, indicating that the second phase at this point is Fe-rich  phase. phase,  phase and Laves phase are also found in the HAZ of K-TIG junction of high frequency pulse.Different from WZ, another precipitating phase can be observed, as shown in Fig. 11.It can be seen that the ellipsoidal rod-like precipitates are evenly distributed in the grain boundary and in the crystal.Generally, the temperature of HAZ during welding is greater than 900℃.According to Fig. 7, it can be seen that carbides or nitrides are likely to be produced under this temperature condition.Yan et al. [37] found the existence of NbC in the grain boundaries and crystals of ferrite in the study of FSS containing Nb. Saha et al. [38] found that during high temperature cooling of FSS, undissolved C elements will form (Ti,Nb)C with Nb and Ti, and (Ti,Nb)C can play a role in dispersion strengthening and hindering sensitization in FSS.Combined with EDS analysis results in Fig. 11(a), it can be seen that the peak of Nb element is high, and the segregation of Nb, C, and Mo elements is found at the location of the sediment through surface scanning (Fig. 11 NbC can also play a dual role of pinning grain boundaries and dragging grain boundary movement, which is another reason for the limitation of HAZ grain growth.In order to further verify the microstructure of the welded joint, phase analysis of WZ and HAZ of AISI444 FSS was conducted by XRD, as shown in Fig. 12.Although the smoothed peaks are easier to observe, in order to ensure the original data, we did not do so.By comparing the XRD diffraction pattern with the PDF card, the peak values and positions of each phase are shown in Table 3.Through observation, it can be seen that the phase composition of WZ and HAZ is very similar, mainly consisting of Fe - as the matrix, and its peak value is large.Jade9 software finds that there are several peaks within these strong peaks.There are also second phases such as  phase,  phase and Laves phase precipitated in the matrix, and their peak value is small due to the content and size of the second phase.In addition, HAZ contains a small amount of NbC.
The results of XRD phase analysis of the precipitated phase are basically consistent with the previous analysis.

Tensile strength
Fig. 13 shows the tensile curves of the welded joints of BM and No.6.Generally speaking, the tensile curves of steel is divided into continuous transition type, uniform yield type and non-uniform yield type.By observing the stretching curve,, it can be found that the tensile curves curves undergo four stages in turn: elastic deformation, plastic deformation, non-uniform plastic deformation and fracture, so both belong to uniform yield type.The tensile strength of the BM is 795 MPa and the elongation is 26.97%.The tensile strength of No.6 joint is 561MPa, reaching 70.57% of the base material, and the elongation is 19.78%.It is found that although the tensile strength of welded joints is lower than that of BM, it is still much higher than that of conventional fusion welding process and achieves higher efficiency [23]. .Compared with BM, the dimples in broken welds are smaller and more densely distributed, which will lead to the reduction of elongation.The decrease of tensile properties is mainly related to precipitation phase and grain coarsening.Hui et al. [31] believe that brittle-hard phase precipitation will lead to the decrease of impact toughness, tensile strength and plasticity at room temperature.

Table 2 .
Process parameters of high frequency pulse K-TIG welding AISI 444

Fig. 2
Fig. 2 AISI 444 FSS under different welding parameters high frequency pulse K-TIG weld appearance

Fig. 4 3. 2 Microstructure 3 . 2 . 1
Fig. 4 AISI 444 FSS high frequency pulse K-TIG weld appearance under different welding parameters faster.Combined with the Schaeffler structure diagram above, the equilibrium phase diagram simulated by JMatPro 7.0 software and the TTP diagram, it is predicted that the AISI444 FSS joint structure is mainly ferrite structure, accompanied by  phase,  phase, Laves phase and '  phase equal precipitation.

Fig. 9
Fig.9 Microstructure and EDS analysis of precipitated phase in the WZ of high frequency pulse K-TIG joint：(a)  phase; (b)  phase.

Fig. 10 EDS
Fig.10 EDS analysis of precipitated phase in the WZ of high-frequency pulse K-TIG joint：(a)~(b)

Fig. 11 EDS
Fig.11 EDS analysis of precipitated phase in the WZ of high-frequency pulse K-TIG joint： (a) Mixed (c)~(i)), and the sediment is finally determined to be NbC.In addition, NbC may also come from BM, as shown in Fig.11(b).In FSS, Nb can improve the strength and affect the tough-brittle transition temperature.Especially, the addition of Nb can preferentially combine with C to form NbC and reduce the intergranular corrosion tendency of FSS.Although HAZ can reach a maximum temperature of 1100℃, NbC has a very high melting point and is difficult to dissolve.

Fig. 12
Fig.12 EDS analysis of precipitated phase in the HAZ of high-frequency pulse K-TIG joint：(a) EDS

Fig. 12 XRD
Fig. 12 XRD patterns of high frequency pulse K-TIG WZ and HAZ

Fig. 13
Fig. 13 Tensile curves of test 6 welded joints and BM

Fig. 14
Fig. 14 Tensile fracture morphology of BM.(a) Macroscopic fracture morphology of BM; (b) Shear lip region and fiber region of BM fracture; (c) Partial enlarged view of (b); (d) Partial enlarged view of (c).

Table 3 .
XRD results of precipitated phase