Thermodynamic study on welding wire design of high nitrogen austenitic stainless steel

Ming Zhu, Wang Kehong, Qu Tianpeng, Wang Wei and Feng Shengqiang

1．School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210054，China;2．Inner Mongolia Institute of Metal Material Research, Baotou 014001, China;3．School of Iron and Steel, Soochow University, Suzhou 215021,China

Abstract Based on thermodynamic calculations, the effect of pressure and alloying elements on the nitrogen content, solidification mode, and welding characteristics were investigated in this study. By increasing the partial pressure of N2, the nitrogen content in the weld pool increased dramatically, and the γ zone was enlarged. The nitrogen content increased as alloying elements such as Cr and Mn were added to the molten steel. The δ zone with high temperature treatment was compressed by adding Ni. These alloying elements play important roles in the formation of the single γ region at the temperature of 298 K. With proper Mn addition, the phase area of γ was extended and became more stable, and the “ferrite trap” was also avoided. Two kinds of welding wires with different nitrogen contents were developed and corresponding MIG welding experiments were performed. As the nitrogen content in wire was higher than that in the base metal, severe blowhole defects and mixture microstructure of δ and γ developed.

Key words high nitrogen steel, nitrogen loss, thermodynamics calculation, wire design

0 Introduction

High nitrogen steel is defined as steels where the nitrogen content exceeds the limitation of materials prepared under atmospheric pressure conditions. Single austenitic-phase microstructure can be obtained at room and low temperatures by partial or complete substitution of N for the alloying element Ni[1]. Because of its excellent comprehensive mechanical properties (high strength, plasticity, and toughness), high nitrogen steel can be widely used in electronics engineering, precision instrument fabrication, and cryogenic processing[2-5].

During the atmospheric welding process, the nitrogen atoms in high nitrogen steel escape from the molten pool in the form of nitrogen gas, decreasing the nitrogen content of the weld metal compared to that of the base metal and the strength coefficient of the weld area is lower. Applications of high nitrogen steel strongly depends on its welding characteristics[6-10], thus, welding technology is important for the development and application of high nitrogen steel. Previous studies[11-12]have shown that suitable laser welding conditions and fast cooling can partially prevent nitrogen loss and nitrogen porosity in high nitrogen steel weld areas. However, the control of the microstructure and mechanical characteristics remains difficult under laser welding conditions. For other welding processes, nitrogen loss in the weld zone is difficult to overcome, and defects such as welding cracks and nitrogen precipitation should also be considered[13]. To date, studies on welding technology for high nitrogen steel welding wires have been inadequate. In this study, the nitrogen behavior in the molten pool of high nitrogen steel and the influence of typical chemical components in welding wires on the equilibrium phase transformation process was studied. The main technical difficulty of using nitrogen alloyed austenitic stainless steel welding wires is the design of appropriate alloy systems to maximize the solubility of nitrogen in the molten pool and improve its solid nitrogen content. The use of the thermodynamic software Thermo-Calc for the study of austenitic stainless steel for nitrogen alloying has been gradually recognized. In this study, the effects of the main alloying elements in high nitrogen steel wire on nitrogen solubility, phase transitions, and precipitate phases during solidification were analyzed using Thermo-Calc thermodynamic software. This was performed to provide theoretical support for the design of high-nitrogen austenitic stainless-steel wire that is low-cost, non-magnetic, and high-performance.

1 Thermodynamic analysis of nitrogen dissolution behavior in steel

The reaction of nitrogen dissolving in the molten pool and related thermodynamic behavior can be described by Eqs.(1)-(3).

(1)

lgKN=-518/T-1.063



ΔG0=9 916+20.17T

(2)

(3)

whereKNrepresents the equilibrium constant,Trepresents the temperature,ΔG0represents Gibbs free energy,fNrepresents the activity coefficient.

According to Sieverts law, as shown in Eq.(4), the Fe-N system was assumed to be an infinite dilute solution, and the activity coefficient,fN, of N in the molten iron is approximately 1, while the influence of other alloying elements was neglected.

(4)

According to Eq.(4), the nitrogen content in the molten pool and N2partial pressure in the protective gas are in thermodynamic equilibrium. Therefore, the nitrogen content in the molten pool can be significantly improved during high nitrogen steel welding by increasing the nitrogen ratio in the shielding gas. However, it is necessary to consider the stability of the welding arc in the actual welding process. In the solidification process of the weld pool, the thermodynamic formula of the solubility of nitrogen[14]in different solidified materials can be described as follows:

lg[%N]=-518/T-1.063 Fe(l)

lg[%N]=-1 520/T-1.04 δ-Fe

lg[%N]=450/T-1.995 γ-Fe

lg[%N]=-1 520/T-1.04 α-Fe

(5)

Accordingly, the solubility curve of nitrogen in different materials as a function of temperature is shown in Fig.1.

Fig.1Solubilityofnitrogenindifferentsolidifiedmaterials

With decreasing temperature, the δ-ferrite phase precipitates first from the liquid phase and the nitrogen solubility decreases sharply. Subsequently, the solubility of nitrogen in the solid phase increases rapidly after the transformation of austenite. Then, with decreasing temperature, the nitrogen solubility decreased significantly after transitioning into the α-ferrite phase transition zone.

2 Effect of typical alloying elements

2.1 Effect of Cr content

With changing Cr content, the saturated solubility of nitrogen in molten steel was significantly affected. The equilibrium phase diagram obtained at different Cr contents is shown in Fig.3, and it is clear that the saturated solubility of nitrogen in the liquid phase increases to a maximum during the initial stage of solidification with decreasing temperature of molten steel. Meanwhile, the saturated solubility of nitrogen increased from 0.63 to 1.29% as the Cr content in the steel increased from 17% to 25%.

2.2 Effect of Mn content

The effect of Mn content of the steel on the equilibrium phase diagram is shown in Fig.3. With increasing Mn content, the saturated solubility of nitrogen in liquid steel increases gradually and can reach 1.2%, when the content of Mn was approximately 20%. Moreover, with increasing Mn content, the negative effect of eutectoid δ-ferrite on the solubility of nitrogen was negligible during the initial stage of solidification. When the content of Mn reached 16%, the solubility of nitrogen is no longer reduced by the precipitation of the high temperature δ-phase and the increase in nitrogen content in the molten steel was not hindered. The most beneficial effect of Mn in nitrogen alloyed stainless steel is the expansion and stabilization of the austenite phase region, and promotion of nitrogen solubility in stainless steel.

Fig.2EffectofCrcontentonthesaturatedsolubilityofnitrogen(a) 17% (b) 22% (c) 25%

Fig.3EffectofMncontentonthesaturatedsolubilityofnitrogen(a) 0% (b) 5% (c) 10% (d) 16% (e) 20%

Inevitably, austenitic stainless steel should undergo the “ferrite trap” during the solidification process at 1 200-1 500 ℃[15]. Therefore, nitrogen loss can lead to local defects in the weld area, and a large number of subcutaneous bubbles can lead to decreased weld strength during the solidification process of high nitrogen steel. The composition design of high nitrogen steel wire must prevent the appearance of “ferrite trop” during post weld solidification. In essence, the “ferrite trap” is the eutectoid process of the δ+γ phase during the first stage of solidification, which is caused by the lack of austenite elements. The variation curve of the ferrite trap region with different Mn contents is shown in Fig.4. It is clear that the area of the “ferrite trap” can be effectively reduced, and the nitrogen in the weld does not escape during the solidification process with increased Mn content in the welding wire. Thus, the mechanical properties of the weld zone are enhanced and improved.

2.3 Effect of Ni content

It is well known that Ni can effectively stabilize the austenite region, and the balance phase diagram with different Ni content is shown in Fig.5. With increasing Ni content in the steel, the γ-phase region expanded gradually and that of the (δ+γ)-phase decreased. Simultaneously, the (L+γ)-phase region enlarged and the δ-and (L+δ+γ)-phase regions compressed. In the actual production process, a certain amount of Ni is beneficial for the formation of the (δ+γ)-phase during the rapid solidification of the weld zone, reducing the δ-phase content which is typically formed in the initial stages of solidification. The solid solution nitrogen content in the weld zone can be significantly improved by the realization of a single γ-phase solidification mode at high temperatures. However, to limit production costs, the content of Ni in high nitrogen steel should be somewhat limited.

Fig.4EffectofMnontheferritetrap

Fig.5EffectofNicontentonsaturatedsolubilityofnitrogen(a) 0% (b) 2% (c) 4% (d) 6% (e) 8% (f) 10%

2.4 Effect of Mo content

The effect of Mo content in the steel on the equilibrium phase diagram is shown in Fig.6. It is clear that the γ-phase region decreased and the (intermetallic+σ)-phase region expanded gradually with increasing Mo content. This has detrimental effects on the corrosion resistance and mechanical properties of steel, resulting in decreased plasticity and toughness. Therefore, the content of austenitic elements, including Ni, N, and Mn, should also be increased with increasing Mo content. It is important to maintain a balance between ferrite and austenite elements in the steel to maintain the single γ-phase structure of high nitrogen steel.

Fig.6EffectofMocontentonthesaturatedsolubilitynitrogen(a) 0.5% (b) 2.0% (c) 6.0%

3 Welding technology and development of welding wires

Based on the thermodynamic analyses above, two types of high nitrogen steel welding wires with different nitrogen contents were developed, as shown in Table 1. The 6 pass MIG welding process was performed on a high nitrogen steel base material with a thickness of 20 mm. The conditions included a welding voltage of 30 V, current of 180-270 A, shielding gas of Ar+N2, gas flow rate of 20 L/min, and welding speed of 180-300 mm/min.

After welding, the weld metal porosity was measured, and the results are shown in Fig.7. No nitrogen hole defects were observed when the nitrogen content in the welding wire was 0.58%. The base metal (approximately 0.75 wt.%) was mixed with nitrogen in the welding wire at a ratio of approximately 30%. The average nitrogen content of the base metal was lower than the maximum saturation in the liquid phase, so the nitrogen could not escape from the molten pool in the form of nitrogen bubbles (Fig.7a). When the nitrogen content of the welding wire was 0.85%, the mixed nitrogen content in the weld pool became higher than the average nitrogen content of the base metal and that of the saturation solubility of nitrogen in the welding pool. Nitrogen bubbles were observed (Fig.7b). To avoid the formation of nitrogen bubble defects in the weld, it was necessary to adjust the nitrogen content in the welding wire or suppress nitrogen bubble formation by increasing the cooling rate.

Fig.7Nitrogengasholetestingattheweldseam(a)No.1weldwire(b)No.2weldwire

To determine the single γ-phase structure, the weld zones after welding by two types of wires were analyzed by X-ray diffraction (XRD) and the results are shown in Fig.8. The weld zone was characterized by uniform γ-phase structure using the No.1 weld wire, and a small amount of ferrite was formed in the weld zone with the No.2 weld wire. This was not useful in completely eliminating the magnetism of the weld seam. In comparison, the No.1 wire exhibited a positive influence on eliminating nitrogen bubbles and the magnetic field in the weld zone.

Table1Typicalchemicalcompositionsofweldmetalandthedevelopedwires(wt%)

SampleChemical compositionsCSiMnCrNiMoNBase metal0.1060.43315.8821.601.800.0260.75Weld wire No.10.0430.34514.0618.911.590.0100.58Weld wireNo.20.0330.11818.0822.212.250.9100.85

Fig.8XRDresultsofweldphasecomposition(a)No.1weldwire(b)No.2weldwire

4 Conclusions

(1)During welding of high nitrogen steel, the weld pool surface partial pressure of N2can be improved via increasing the proportion of N2in the shielding gas. Simultaneously, the solubility of nitrogen in the molten pool can be significantly improved and the nitrogen content of weld can also be improved. In addition, the γ-phase region can be expanded by controlling the single γ-phase solidification mode.

(2)The alloying elements exert a significant influence on the saturation solubility of nitrogen in stainless steel and on the solidification process of molten steel. Cr mainly increases the solubility of nitrogen in liquid steel. A small amount of Ni increased the minimum nitrogen solubility of the molten pool at high temperatures and reduced the temperature range of high temperature δ-ferrite phase, which is conducive to the formation of γ-phase structure at room temperature. With increasing Mn content, the solubility of saturated nitrogen in liquid steel and the minimum nitrogen solubility during the initial stage of solidification increased. An appropriate Mn content can expand and stabilize the austenite phase area, preventing the appearance of the “ferrite trap”.

(3)Two types of high nitrogen steel wires were tested in MIG welding experiment. While the content of nitrogen in the welding wire is higher than that of the base metal, more obvious porosity defects were observed in the weld zone and the solidification structure was a mixture of ferrite and austenite. The welding wire with a nitrogen content of 0.58% was determined to be more suitable for the welding of high nitrogen steel.