Welding and Joining
Effect of Brazing Temperature on Microstructure and Mechanical Property of High Nitrogen Austenitic Stainless Steel Joints Brazed with Ni–Cr–P Filler
2019 Volume 59 Issue 2 Pages 300-304
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2019 Volume 59 Issue 2 Pages 300-304
In this study, Ni–Cr–P filler was used for vacuum brazing of high nitrogen austenitic stainless steels (HNS). The microstructure and shear strength of HNS joints were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD) and universal testing machine. The results exhibit that Cr2N compounds are formed at the HNS/filler interface when the brazing temperature is lower than 1000°C. The brazing seam is composed of Ni–Fe solid solution and (Ni, Cr)3P compounds, and the content of (Ni, Cr)3P compounds decrease with the increasing of brazing temperature. The formation of Cr2N and (Ni, Cr)3P compounds is adverse to the joint strength due to their brittleness. The optimal shear strength of joints is 163 MPa when the brazing temperature is 1050°C.
High nitrogen austenitic stainless steels (HNS) are a promising structural materials which can obtain the stable austenite by using nitrogen as alloying element.1,2) The addition of nitrogen improves its strength and corrosion resistance without the sacrifice of ductility or toughness at the same time.3) Therefore, HNS possesses great potential to take place of the widely used 304 or 316 austenitic stainless steels for application in many fields, such as biomedicine, automobile parts, ocean engineering, energy, chemical and military industries.4,5) However, welding and joining is an important issue during the process of industrial application of HNS.
Fusion welding such as shielded metal arc welding,6,7) gas tungsten arc welding,8,9) laser welding,10,11) is widely used for joining HNS. However, several shortcomings can often be observed during fusion welding, such as N2 pores and hot cracking. Therefore, it is indispensable to choose the type of filler metal or flux and control the welding parameters carefully. Friction stir welding (FSW) and friction welding are both solid state welding processes, which are favorable for decreasing the nitrogen loss and improving the strength of HNS joints.12,13) However, friction welding has certain limitations for joints configuration.
Brazing is an appropriate method to join complex shape components,14) which is favorable for enhancing the design space for components and structures. In addition, brazing is free of the usual defects caused by fusion welding. However, to the best of our knowledge, there are very limited studies on brazing of HNS.15) Ni-based filler metals exhibit excellent wettingability on the surface of traditional austenitic stainless steel and provide joints with excellent corrosion resistance and mechanical property at elevated temperature.16,17) In the previous study,15) we found that the B from the Ni–Cr–B–Si filler reacted strongly with the N from the HNS substrate to form BN compounds, which play a disadvantageous role on the joint strength.
In this paper, the Ni–Cr–P filler metal without B was used for vacuum brazing HNS. Phosphorus in Ni–Cr–P filler metal can reduce the melting point while chromium provides high oxidation resistance. The current study attempts to investigate the effect of brazing temperature on the microstructure and shear strength of the brazed joints.
The chemical composition of HNS used in this study in mass percent was 0.04C, 18.5Cr, 17.9Mn, 1.8Mo, 0.71N, 0.25Si, 0.035P, 0.022S and balance Fe. The tensile and shear strength of HNS substrate were 1050 MPa and 580 MPa, respectively. The microstructure and XRD pattern of the used HNS base metal is shown in Fig. 1. It can be seen that the HNS consists of fine equiaxed grains of austenite and annealing twins. In order to obtain the Time-Temperature-Precipitation (TTP) curve of nitride precipitation, the HNS substrates were isothermally annealed at temperatures of 900°C, 850°C, 800°C and 750°C for various time. The quantitative metallographic analysis with optical microscope (OM) and Image-pro Plus (IPP) 5.0 software was used to detect the content of nitride based on the previous studies.18,19) It is well known that the Cr2N initially precipitate discontinuously along the grain boundaries. Therefore, the phase only located at grain boundary was identified as Cr2N. However, this method of quantitative metallographic analysis exists a certain error. Figure 2 exhibits an example of calculating the content of Cr2N by using the OM image and IPP software according to the metallographic principle that the volume fraction is equal to the area fraction. At least five different positions were measured for each sample.
OM and XRD results of HNS substrate.
OM image of HNS aged at 850°C for 5 min (a), the nitride were signed as red by IPP software (b). (Online version in color.)
The Ni–Cr–P filler alloy in the form of powder was used as filler metal. The average diameter of the filler powder was about 50 μm. The nominal chemical composition of the Ni based filler in weight percent was 13–15Cr, 9.7–10.5P, and the balance was Ni.
The faying surfaces were ground by 1000# SiC paper and ultrasonic cleaned in acetone for removing the oxide film and greasy dirt. The filler powder was mixed with 3 wt.% ethyl cellulose as binder to obtain the filler pastes for fixing the joining specimens. The schematic diagram of joints assembly was shown in Fig. 3. The sandwich assembly was heated at the rate of 15°C/min to brazing temperature (950°C, 1000°C and 1050°C) and held for 20 min under a vacuum of 10−3 Pa, and then furnace cooled to room temperature. No pressure was applied during the brazing process.
Schematic diagram of joint assembly.
Microstructure and element distribution of the brazed joints were analyzed using the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The phase composition of joints was identified by X-ray diffraction (XRD). Microhardness test was performed to determine the hardness distribution across the brazed joints. The test was conducted on samples cross sections with a constant load of 100 g for 10 s. Shear testing was carried out by using a universal testing machine at 0.25 mm/min crosshead speed. All shear testing results were an average of three samples for each condition.
It is shown in Fig. 4 that the microstructure of HNS joint brazed at 950°C for 20 min. No pores or cracks are formed in the joints, indicating excellent wettingability between the HNS substrate and Ni–Cr–P filler. It can be seen that the continuous stripe phase is formed in the center of brazing seam. At the same time, a small quantity of spot-shaped phases is found in the brazing seam near the HNS/filler interface. In addition, a layer of cellular phase is formed from the HNS/filler interface and grows towards the base metal.
SEM images of joints brazed at 950°C for 20 min.
Chemical composition of the special phases is analyzed by EDS and the results are presented in Table 1. The XRD analysis was performed to further identify the phases formed and the results are shown in Fig. 5. The EDS results show that the light-grey phase (marked as 1) is rich in Fe, Cr and Ni. According to the Ni–Fe, Ni–Cr phase diagram,20) the solubility of Fe and Cr in Ni is about 19% and 40%. So, the point 1 can be identified as Ni based solid solution (denoted as Ni (s, s)). The existence of Fe in brazing seam can be attributed to the dissolution of HNS. The significant enrichments of Ni, Cr and P are detected in the dark-grey phases (marked as 2) which are regarded as (Ni, Cr)3P compounds according to the XRD pattern (Fig. 5(I)). The spot-shaped phases (marked as 3) are considered to be (Fe, Cr)3P due to high content of Fe, Cr and P. However, the (Fe, Cr)3P phase has not been detected by XRD pattern due to their low content. Finally, the point 4 and 5 can be identified as Cr2N and γ-Fe phase based on the EDS results in Table 1 and XRD pattern in Fig. 5(II).
| Position | Fe | Cr | Mn | Mo | N | Ni | P | Possible phase |
|---|---|---|---|---|---|---|---|---|
| 1 | 30.24 | 14.41 | 6.34 | 0.11 | 0 | 47.49 | 1.4 | Ni (s, s) |
| 2 | 11.9 | 28.94 | 7.39 | 0.38 | 0 | 28.18 | 34.61 | (Ni, Cr)3P |
| 3 | 28.7 | 48.02 | 5.15 | 0.63 | 0 | 8.09 | 25.19 | (Fe, Cr)3P |
| 4 | 47.21 | 18.27 | 16.27 | 0.12 | 17.35 | 0.67 | 0.11 | Cr2N |
| 5 | 54.53 | 18.85 | 19.42 | 0.28 | 5.75 | 0.77 | 0.24 | γ-Fe (s, s) |
XRD patterns of HNS joints brazed at 950°C for 20 min, the corresponding analyzed positions were marked in Fig. 4.
Previous study has shown that the nitrogen alloyed austenitic stainless steel is easy to precipitate Cr2N at temperatures ranging from 700 to 1000°C.21) However, no obvious precipitation of Cr2N is observed in the base metal probably because of short holding time and fast cooling rate. Figure 6 shows that the typical cooling curve of brazed joints and the TTP curve of HNS substrate which starts with precipitation of 0.1 vol.% Cr2N. The cooling rate of brazed joints is variable for different temperature ranges. The key cooling rate is from the peaking temperature of heating cycle (e.g. 1000°C) to the nose temperature of TTP curve (850°C), which is about 45°C/min. Furthermore, the critical cooling rate for avoiding formation of Cr2N can be approximately calculated by Eq. (1):22)
| (1) |
The cooling curve of brazed joints and TTP curve of Cr2N precipitation.
Where Vc is the critical cooling rate, Tp is the peaking temperature of heating cycle, Tn is the nose temperature, ti is the incubation period. According to the TTP curve, the nose temperature (Tn) and incubation period (ti) is 850°C and 5 min, respectively. In thus, when the brazing temperature is 1000°C, the critical cooling rate is 30°C/min. Therefore, the cooling rate of brazed joints is higher than the critical cooling rate. At the same time, Fig. 6 also exhibits that the cooling curve of brazed joints does not intersect with the TTP curve. As a result, the precipitation of Cr2N in the HNS substrate during brazing can be avoided. On the contrary, the Cr2N compounds started to nucleation at the HNS/filler interface and grow toward the base metal due to high interface energy.
The effect of brazing temperature on the microstructure of joints is shown in Fig. 7. It can be seen that the content of Cr2N compounds at the HNS/filler interface decreases with the increasing of brazing temperature. When the brazing temperature is 1050°C, the Cr2N compounds completely disappear. The reason is that Cr2N will dissolve into the austenite at high temperature according to the Fe–Cr–Mn–N phase diagram.23) At the same time, this also proves that the Cr2N cannot be formed during the cooling process due to low critical cooling rate. Furthermore, the content of (Ni, Cr)3P phases also decreases with the increasing of brazing temperature. High brazing temperature promotes the diffusion of P element into the HNS substrate. However, it is difficult to eliminate the brittle (Ni, Cr)3P compounds completely due to slow diffusion rate of P element.24) In addition, the distributions of main elements were measured by EDS along the white base line, as shown in Fig. 8. On the one hand, a large amount of Fe and Mn elements dissolves into the filler due to high solubility between Ni–Fe and Ni–Mn.20) On the other hand, a small quantity of Ni and P elements diffuses into the base metal.
SEM images of joint brazed at 950°C (a), 1000°C (b), 1050°C (c) for 20 min.
Elemental distributions of Ni, Fe, Mn, Cr and P across the brazed joints.
Figure 9 shows that the microhardness across the joints brazed at different temperature. The maximum hardness in the center of brazing seam can be attributed to the formation of brittle (Ni, Cr)3P compounds. The hardness of cellular phase is also higher than that of substrate metal, which indicates the brittleness of Cr2N compounds. However, the Ni (s-s) phases exhibit similar microhardness value to the HNS substrate. With the increasing of brazing temperature, the width of high hardness zone decreases, which is consistent with the analysis of joint microstructure.
Microhardness distribution across the HNS joints brazed at 950°C (a), 1000°C (b) and 1050°C (c).
The effect of brazing temperature on the shear strength of brazed joints is shown in Fig. 10. It is clear that the shear strength increases with the increasing of brazing temperature. The maximum shear strength of the joint can reach 163 MPa, which is up to 28% of the shear strength of the HNS substrate. The SEM images of the fractured surface are shown in Fig. 11. The three specimens all exhibit the brittle feature. The Cr2N and (Ni, Cr)3P compounds can be observed in the fracture surface. Therefore, the formation of brittle Cr2N and (Ni, Cr)3P compounds is adverse to the joint strength. Similarly, Wu et al.25) reported the optimal shear strength of super-Ni/NiCr laminated composite/Ni–Cr–P/Cr18-Ni8 steel joint was only 137 MPa due to the formation of brittle Ni3P. Wang et al.26) also reported that the shear strength of the Mo–Cu/Ni–Cr–P/304 stainless steel joint was about 155 MPa because of the formation of brittle Ni5P2 and Cr3P.
Shear strength of joints brazed at different temperature.
SEM images of fracture surface of HNS joints brazed at (a) 950°C, (b) 1000°C and (c) 1050°C.
The maximum joint strength obtained by brazing is lower than that by melting welding or FSW,11,12,13) which is an inherent deficiency of brazing method.27) However, each joining technology has its own advantages and disadvantages. The advantage of brazing HNS with Ni–Cr–P filler can be summed up in the following aspects. Firstly, brazing of HNS is more appropriate for fabricating complex shaped parts. Secondly, no defects such as pores or cracks are observed in the brazed joints. However, nitrogen pores can easily occur in the weld metal during fusion welding of HNS due to the difference of nitrogen gas solubility between molten and solid metals.28) Thirdly, the brazing process with Ni–Cr–P filler at 1050°C has no effect on the microstructure and phase composition of HNS substrate. The precipitation of Cr2N in the HNS substrate during the cooling process of brazing can be avoided. At the same time, the transformation between γ-ferrite and δ-ferrite cannot occur. The reason is that the peaking temperature of brazing is lower than the phase transition temperature.29) On the contrary, precipitation of δ-ferrite can be observed in the stir zone of FSW joints13) and the HAZ of fusion welding joints29) due to high heating temperature. Brazing of HNS with B containing filler leads to formation of BN compounds at the interface and causes precipitation of δ-ferrite from HNS substrate.16)
Brazing of high nitrogen austenitic stainless steels (HNS) was carried out by using Ni–Cr–P filler. The effect of brazing temperature on the microstructure and shear strength of joints was investigated. The results show that the (Ni, Cr)3P compounds are formed in the center of brazing seam and Cr2N compounds are precipitated at the HNS/filler interface. With the increasing of brazing temperature, the content of (Ni, Cr)3P and Cr2N decreases and the shear strength of joints increases. The optimal shear strength of joints is 163 MPa when the joining temperature is 1050°C due to no formation of Cr2N at the interface and low content of (Ni, Cr)3P compounds. The most important advantage of brazing with Ni–Cr–P filler is that the brazing process has no effect on the microstructure and phase composition of HNS substrate.
This work was financially supported by the Scientific Research Project of the Education Department of Jilin Province (JJKH20181024KJ). The author thanks Prof. B. S. Wang for helpful discussion.
