Problems in Welding of High Nitrogen Steel: A Review

Abstract

High nitrogen steel (HNS) has an excellent tensile strength and impact property due to the solid solution strengthening of nitrogen, which provides a good application prospect in many fields. Fusion welding is one of the main processing methods of HNS, but the process is prone to spatters, serious nitrogen losses, N₂ porosities, and poor performances of joints, resulting in the failure of large-scale engineering applications of HNS. In this work, the development of HNS welding is reviewed, and the problems including the droplet transfer instability, N₂ porosities, nitrogen losses and poor mechanical properties due to high nitrogen content are discussed. According to previous welding experiences, the adoption of a welding method with a low heat input is proposed, which utilizes powders instead of wires, optimizes compositions in the shielding gas and feeding materials in order to solve the above problems, and improves the mechanical properties of the weld.

1. Introduction

In the 1950s, or even earlier, a lack of nickel resulted in the use of nitrogen as a replacement to stabilize austenite, and HNS came into being [1]. Steel with nitrogen contents that exceeded the limit value of austenitic steel under conventional smelting conditions was called HNS, and the mass fraction of nitrogen was generally greater than 0.4%. Nitrogen is a kind of strong austenite stabilizing element. In addition to stabilizing austenite and inhibiting the production of ferrite, it also has a stronger solid solution strengthening effect than nickel, which can significantly improve its tensile strength, generally reaching the level of 1100 MPa, which is much higher than that of ordinary austenitic stainless steel [1,2]. HNS also has good compatibility with the human body, which can effectively avoid the human body’s allergy to nickel [3]. In conclusion, HNS not only has the characteristics of general stainless steel, but also has high tensile strength and corrosion resistance, which makes it have a good application prospects in aerospace, shipbuilding, offshore construction, weapon manufacturing, medical devices and other fields [4,5]. Fusion welding is one of the main processing methods of HNS. However, the fusion welding process of HNS is prone to spatters, poor forming, serious nitrogen losses, N₂ porosities, poor performance of joints and other problems, resulting in the failure of the large-scale engineering application of HNS. In this work, the development of the HNS welding is reviewed, and the problems including the droplet transfer instability, N₂ porosities, nitrogen losses and poor mechanical properties due to high nitrogen content are discussed. According to previous welding experiences, the adoption of a welding method with a low heat input is proposed, which utilizes powders instead of wires, optimizes compositions in the shielding gas and feeding materials to solve the above problems, and promotes the development of the HNS welding technology and the engineering application of HNS.

2. Instability of Droplet Transfer

During the welding of HNS with wires melted by the heat source, the molten welding wire forms droplets. The solubility of nitrogen in the solid phase (austenite region) is much higher than that in the liquid phase and Nitrogen has the highest solubility in the austenite region [6]. The solubility of nitrogen in the droplets will drop sharply at the high temperature more than 1500 °C, when the nitrogen content in the droplet is higher than the solubility of the nitrogen at the atmospheric pressure. As a result, a large number of supersaturated nitrogen atoms escape and gather into bubbles. When nitrogen atoms gather into the large N₂, the droplet transfer becomes unstable, and spatter and splashes may occur, as shown in Figure 1a [7]. The stability of droplet transfers has been investigated using the high speed camera technology in detail in the Reference [7]. There are three main problems caused by unstable droplet transfer in welding. One is the loss of nitrogen in HNS welding wires, which eventually leads to the reduction in nitrogen contents in the molten pool. Second, the splash is mainly composed of Mn, Cr, Mu, and their compounds. The loss of these elements will reduce the solubility of nitrogen, which is not conducive to the solid solution strengthening. Third, the spatter may cause slag inclusion during the multi-pass welding and additive manufacturing, and there is a risk of the interlayer tearing. Compared with other arc welding methods, CMT (Cold Metal Transfer) has the smaller heat input and lower degree of the droplet overheating, which can improve the instability of droplet transfer caused by the escape of a lot of nitrogen. Yang et al. [8,9] promoted the formation of the short-circuit transition by controlling the currents and voltages, which was relatively stable, but a splash problem still existed [10,11]. The splash may cause inclusions between layers and lanes, which may lead to the lamellar tearing. Wei et al. [12] used the mixed shielding gas N₂ + Ar to weld the HNS plates, and found the disordered N₂ in the arc atmosphere disturbed the stable combustion of the arc with the increase in the N₂ content in the shielding gas, resulting in more serious splashes. Figure 2 shows the influence of different N₂ contents in the shielding gas on spatters in welding. It should also be noted that a large amount of supersaturated nitrogen escaped during the droplet transfer, which will lead to the loss of nitrogen in the whole welding system, and also have a great impact on the mechanical properties.

3. Porosity Defect

The N₂ porosity is the most common defect in the welding of HNS, and the formation process is shown in Figure 1b [13]. Firstly, the molten pool is solidified with the decreasing temperature, and the solubility of nitrogen in the solid phase is much lower than that in the liquid phase [14]. Therefore, a large number of nitrogen atoms will escape and gather to form the large molecules of N₂. Then, N₂ floats up under the action of buoyancy. When the crystallization speed of the molten pool is greater than the floating speed of N₂, the N₂ porosity defects are formed in the crystallized solid phase. Moreover, the solubility of nitrogen in the ferrite phase is very small, thus nitrogen is gradually excluded from the ferrite-dendrite phase during cooling, which leads to a nitrogen-rich zone. With the continuous accumulation of nitrogen, it is easy to form N₂ porosities [13].

3.1. Effect of the Shielding Gas

Through adjusting the composition and content in the shielding gas, the formation of N₂ porosities can be effectively controlled. When an appropriate amount of N₂ is added into the shielding gas, the accumulation of nitrogen atoms in the molten pool can be effectively inhibited, and the formation of N₂ porosities can be reduced [15,16,17]. As shown in Figure 3, the porosity ratio shows a tendency to decrease first and then increase with the increase in the N₂ content in the Ar+N₂ shielding gas, and the porosity ratio is the lowest 0.25% under the shielding gas of 90%Ar + 10%N₂ [17]. If the nitrogen content in the shielding gas is too high, the supersaturated nitrogen atoms in the molten pool are more likely to combine to form diatomic nitrogen. As a result, the diatomic nitrogen escapes from the molten pool in the form of bubbles, accompanied by splashes and violent metal discharge, which significantly increases the number of porosities [17,18,19]. Cui et al. [17] also tried to add O₂ into the shielding gas. Although the nitrogen content in the weld increased, the N₂ porosity became more serious.

3.2. Effect of Welding Materials

As shown in Figure 4, the high nitrogen content of the welding wire also has an increasing effect on the N₂ porosity [20]. The weld obtained by the 0.46% N wire has almost no porosity. The porosity in the weld obtained by the 0.61% N wire is still very small, except for a small number of porosities found at the arc starting position of the weld, porosities are hardly found in other positions of the weld. The number of porosities in the weld obtained by the 0.84% N wire is much more than those under the 0.46% and 0.61% N wire, and the porosity is evenly distributed in the weld. When using 0.84% N welding wire, the concentration of nitrogen in the molten pool is much higher than the equilibrium solubility of nitrogen, and the probability of supersaturated nitrogen aggregation to form nitrogen increases, resulting in a sharp increase in the number of porosities in the weld.

3.3. Effect of Process Parameters

In addition to the shielding gas and welding materials, process parameters are also one of the main factors affecting porosities. As shown in Figure 5, the number of N₂ porosities reduces with the reduction in the heat input during wire and arc additive manufacturing using the CMT+P model [21]. When the heat input is 180.2 J/mm, porosities are hardly found in the longitudinal section of the additive seam. Compared with the arc welding, the speed of the laser arc hybrid welding is significantly improved, which can effectively inhibit the aggregation of nitrogen atoms during the solidification of the molten pool. The difficulty lies in the need to select the best parameters in a narrow range of the laser arc hybrid welding process parameters. Wang et al. [22] pointed out that the appropriate laser power can effectively control the porosity in the experiment as shown in Figure 6a. Cui et al. [23] indicated that the number of porosities in the weld was decreased with the increase in defocusing amount. The ultrasonic vibration and mechanical vibration can also reduce the porosity during laser arc hybrid welding process [17,22,24]. With the increase in the mechanical vibration frequency, the N₂ porosities first decrease and then increase. The porosities ratio is the lowest 0.44 under the vibration frequency of 35 HZ, as shown in Figure 6b [22]. For the ultrasonic vibration, the same trend of first decreasing and then increasing can be found [17]. Therefore, an appropriate vibration frequency needs to be determined to reduce the formation of porosities.

4. Nitrogen Loss

The basic reason for the high tensile strength and impact toughness of HNS is that the supersaturated nitrogen atoms are embedded in the interstitial space of the iron atom lattice, and the interstitial solid solution is formed in HNS under nitrogen rich conditions, which is conducive to improving mechanical properties. Welding is a rapid heating and cooling process. The base metal and welding materials of HNS have undergone a complex metallurgy process, which generally leads to the reduction in nitrogen contents in the weld, and in particular, the solid solution nitrogen (SSN) is significantly reduced. The solid solution strengthening effect is weakened, and the mechanical properties are seriously affected. Therefore, the high amount of SSN in the weld guarantees high mechanical properties. The reasons for the nitrogen loss in the HNS weld are as follows:

• During the droplet transfer, nitrogen atoms are gathered to form large N₂ molecules, which may cause explosions, and the nitrogen in welding materials is lost.
• In the solidification of the molten pool, the nitrogen escapes and the formation of the N₂ porosity will cause the loss of nitrogen.
• As the molten pool cools to below 1100 °C, the nitrogen is combined with Mn, Cr, Ti and other alloy elements to form nitrides, resulting in the loss of SSN.

Therefore, the loss of nitrogen in welding can be reduced by increasing the nitrogen contents of welding materials, ensuring the stability of the droplet transfer, controlling the formation of the N₂ porosity and inhibiting the formation of nitrides.

4.1. Effect of the Shielding Gas

The improvement of the nitrogen content in the shielding gas can increase the nitrogen content in the weld. As shown in Figure 7, Qiang et al. [12] implied that the nitrogen content of the weld increases with an increase in the N₂ content in the shielding gas Ar + N₂ during double-sided synchronous welding of HNS. It is worth noting that when the N₂ content in the shielding gas is less than 40%, the effect of increasing nitrogen is relatively significant. Cui et al. [17] also found the similar trend of increasing the nitrogen content of the weld with an increase in the N₂ content in the shielding gas. However, the N₂ content in the shielding gas is more than 10%, and the porosity ratio will maintain a high value, as shown in the Section 3.1. When the Ar-N₂-CO₂ is used as the shielding gas to weld HNS, the nitrogen content of the weld also increases with increasing N₂ content in Ar-N₂-CO₂ [25]. To sum up, increasing the N₂ content in the shielding gas can effectively increase the nitrogen content in the weld, but the content should be strictly controlled, and generally should be kept at less than 10%.

4.2. Effect of Welding Materials

Ma et al. [20] implied that the nitrogen content in the weld can be increased by using a welding wire with high nitrogen content, but the excessive nitrogen content in welding wires will correspondingly increase the porosity tendency. In the works conducted by Kumar et al. [26], the above conclusion was also found. As shown in Figure 8, Liu et al. [27] successfully achieved the goal of increasing the nitrogen content in the weld by increasing the nitrogen content in the welding wire. The theoretical values in Figure 8 are calculated by the penetration ratio of 30% (the nitrogen content of the base metal × 30%+ the nitrogen content of the welding wire × 70%). It is not difficult to find that the nitrogen content of the weld increases with the increase in the nitrogen content in the welding wire, but the nitrogen loss also becomes more serious. In Section 3.2, it is pointed out that when the nitrogen content in the welding wire increases to a certain value, the welding porosities will increase significantly, which is not conducive to the mechanical properties as shown in Figure 4. Wu et al. [28] used Cr-Mn-N wires containing 0.79 wt% nitrogen (HNS6) and pure Cr₂N (99.9%) powder to manufacture the straight arm structure. Although a large amount of nitrogen was lost in the manufacturing process, the nitrogen contents in the straight arm could reach 0.99 wt%. It is worth pointing out that the nitrogen contents in the straight arm structure can reach 0.99 wt%, but its tensile strength is only 1000 MPa as much nitrogen exists in a combined state. Liu et al. [29] welded HNS by adding nitride into the molten pool, and filled the MIG molten pool with a tubular filler made of MnN powder as a bypass MnN flux cored wires by a method similar to double wire feeding. The results showed that ferrite contents were decreased by 51.4% and the nitrogen contents were increased by 0.22%, but the tensile strength was only 832 MPa. The addition of Mn promotes austenitization, but the contents of SSN are small, and the formation of many nitrides affects the properties. Therefore, we should not blindly pursue high nitrogen contents, but try to increase the contents of SSN, and reduce the contents of the combined nitrogen as much as possible in experiments. As shown in Figure 1c, the excessive precipitation of Cr-N compounds and Mn-N compounds during the cooling is also the reason for the SSN loss. The thermal sensitivity of nitrides in welds is determined by their compositions. The existence of nitrides not only reduces SSN, but also affects the corrosion resistances and mechanical properties of the weld. Wang et al. [13] pointed out that the thermal sensitive temperature range of nitrides was 850~900 °C, and nitrides would be precipitated when the residence time in this range exceeds five minutes. Therefore, the residence time in the thermal sensitive range should be avoided in practical application.

In Section 2 and Section 3, the reasons for instability of the droplet transfer and N₂ porosities have been explained in detail. With the development of direct energy deposition technology, it is considered that the groove can be filled by the direct energy deposition of the HNS powder, which will not lead to unstable droplet transfer, or even explosion, resulting in ensuring the welding forming and nitrogen contents.

5. Mechanical Property

5.1. Fracture Mechanism

According to the references [27,28], the fracture usually occurs at the weld in the tensile test, and the fracture section presents a ductile fracture feature. During the tensile process, micropores are formed inside the material, and under the action of tensile stress, the micropores grow and merge to form dimples. The size, depth and morphology of the dimples can reflect the plasticity of the welded joint to a certain extent, as shown in Figure 9a,b, and the larger the dimple and the deeper and more uniform distribution, the more this would indicate that the microstructure of high nitrogen steel is uniform and the toughness is better. Moreover, the second phase particles found in the dimples of high nitrogen steel tissues are mostly Chromium ferromanganese oxide inclusions (chromium ferromanganese oxide inclusions) as shown in Figure 9c,d.

5.2. Effect of Welding Methods

The mechanical properties of the weld are generally lower than that of the base metal due to the poor forming, porosity defects and the nitrogen loss in welding, as shown in

Table 1. Mechanical Properties of HNS welded structure.

No.	Processing Method	Tensile Strength/MPa	Elongation/%	Fracture Location	References
1	MIG	900	14.5	Weld bead	[20]
2	MIG	912.5	/	Weld bead	[27]
3	TIG	956.7	6.8	Weld bead	[29]
4	TIG	832	/	Weld bead	[25]
5	CMT+P additive manufacturing	829	14.1	Additive structure	[8]
6	CMT+P additive manufacturing	860.6	34.9	Additive structure	[30]
7	PA additive manufacturing	1078	46.1	Additive structure	[31]
8	Double wire filled PA additive manufacturing	1166	51	Additive structure	[32]
9	Laser-MIG hybrid welding	980.15	/	Weld bead	[34]
10	Laser-MIG hybrid welding	923.7	16	Weld bead	[35]
11	Laser-MIG hybrid welding	928.9	7.3	Weld bead	[36]
12	Wire-powder hybrid additive manufacturing	1014	50.8	Additive structure	[28]

. During MIG welding, the tensile strength is slightly larger than 900 MPa, and the elongation and impact toughness are awfully low [20,27]. During TIG welding, the tensile strength is improved a little, but the performance is still much worse than the base metal [25,29]. During AM (Additive Manufacturing) by CMT, the tensile strength is only 860 MPa lower than that during TIG due to splashing and inclusions caused by unstable droplet transfers [8,30]. During AM by Plasma arc (PA), the tensile strength can reach 1166 MPa, and the elongation can reach more than 50%, these performances are very close to HNS [31,32]. However, the biggest disadvantage of PA processing HNS is the low efficiency. Compared with conventional arcs, the laser not only provides lower heat inputs during welding, avoiding unstable droplet transfers effectively, but also makes the crystallization speed of the molten pool faster, preventing the massive accumulation of nitrogen atoms to form porosities [33]. During laser-MIG hybrid welding, the tensile strength can reach 980 MPa, and the elongation is much lower than that of HNS due to the uneven microstructure in the arc and the laser action areas [34,35,36]. Wu et al. [28] used wire-powder hybrid additive manufacturing technology to fabricate an HNS straight wall structure, the tensile strength of which reaches 1014 MPa, and the elongation of which reaches 50.8% due to the high nitrogen content in wires and powders.

5.3. Effect of the Shielding Gas

The tensile curves of the HNS welded structure with different nitrogen contents in the shielding gas are shown in Figure 10 [37]. When the content of N₂ in the shielding gas is increased from 0 to 20%, the tensile strength is increased from 784 MPa to 852 MPa. However, when the N₂ content in the shielding gas is more than 10%, the porosity ratio will maintain a high value, as shown in the Section 3.1. Therefore, when the N₂ content in the shielding gas is large, it will not improve the mechanical properties, but will promote the formation of porosities and reduce the mechanical properties. Moreover, it is necessary to consider the interaction of multi-gases in the shielding gas. As shown in Figure 11, Liu et al. [25] optimized the composition in the ternary shielding gas Ar-N₂-CO₂, and implied the interaction between CO₂ and N₂ will reduce the mechanical properties. The final optimal shielding gas composition is 87%Ar-6.5%N₂-6.5%CO₂.

5.4. Effect of Microstructures

The ideal microstructure of the HNS structure is austenite, and nitrogen is dissolved in austenite, and the presence of ferrite can lead to a decrease in mechanical properties [38,39,40]. Figure 12 shows the effect of microstructures on mechanical properties of the HNS structure [28]. When the solidification is the AF mode, a small amount of ferrite appears in the microstructure of the additive structure, and the tensile strength and elongation are 877 MPa, 42.2% respectively. When the Cr₂N powder is used to fabricate the HNS additive structure by the wire-powder hybrid additive manufacturing, the solidification changes from the AF mode to A mode, the microstructure of the HNS additive structure presents an all austenite microstructure, and the tensile strength and elongation increases to 959 MPa, 46.5%. In conclusion, the A solidification mode can be controlled by adjusting Creq (Cr equivalent) and Nieq (Ni equivalent) of the welding materials, which can promote the formation of austenite microstructures in the HNS manufacturing structure [41,42,43]. As a result, the mechanical properties of the HNS manufacturing structure can be improved.

5.5. Effect of Welding Materials

According to the Section 3.2 and Section 4.2, the welding materials can affect the porosities, microstructures and nitrogen losses. Thus, the nitrogen content in the welding materials can also affect the mechanical properties. Figure 13 displays the effect of the nitrogen content in welding wires on the tensile strength and impact energy of the HNS welding joint [27]. As the nitrogen content in the welding wire increases from 0.15 to 0.9, the strength increases first and then decreases, and the tensile strength and impact energy reach the highest value 912.5 MPa and 138.17 J under the welding wire of 0.6% nitrogen content. In the ref. [20], a similar trend can be found, and the highest tensile strength is obtained when the nitrogen content is 0.61%. When the nitrogen content in the welding wire is very high, it promotes the formation of porosities in the molten pool, and the nitrogen loss becomes more serious. Therefore, the mechanical properties may be reduced when the high nitrogen content welding wire is used to fabricate HNS. Wu et al. [28] added the pure Cr₂N (99.9%) powder to manufacture the straight arm structure of HNS, increasing the nitrogen content in the straight arm 0.99 wt%, and the tensile strength reaches 1000 MPa.

6. Conclusions

In this work, the development history of the HNS welding is reviewed. The main conclusions are as follows:

• During the droplet transfer, many nitrogen atoms gather into the large N₂, the droplet transfer becomes unstable, and spatter and splashes may occur. The welding methods with the low heat input, such as CMT, PA and laser arc hybrid welding methods, can be used to keep the droplet transfer stable. Besides, using powder instead of welding wires to fabricate HNS in welding can also avoid the formation of droplets, and improve the explosion problem.
• A large number of nitrogen atoms gather into N₂ during the solidification of the molten pool. When the floating speed of N₂ is less than the crystallization speed of the molten pool, the N₂ porosity will occur. A suitable N₂ content in the shielding gas, a suitable nitrogen content in the welding materials, and optimized process parameters can help to avoid the N2 porosities.
• The explosion of molten droplets, the formation of the N₂ porosity, and the precipitation of nitrides are the main reasons for the nitrogen loss in the weld, especially the SSN loss. A proper increase in the nitrogen content in the shielding gas and welding materials can effectively ensure the nitrogen content in the weld. Besides, accelerating the cooling rate in the nitride thermal sensitive temperature range can effectively inhibit the precipitation of nitride.
• The mechanical properties of the weld are generally lower than that of the base metal due to the droplet transfer instability, N₂ porosities and nitrogen losses in welding. Stabilizing the droplet transfer, inhibiting N₂ porosities and reducing nitrogen losses are the means to ensure better mechanical properties. Furthermore, the A solidification mode can promote austenite in the HNS manufacturing structure, which can improve mechanical properties.