Austenitic Stainless Steel Powders with Increased Nitrogen Content for Laser Additive Manufacturing

Abstract

Nitrogen is used as an alloying element, substituting the expensive and allergenic element nickel, in austenitic stainless steels to improve their mechanical properties and corrosion resistance. The development of austenitic stainless steel powders with increased nitrogen content for laser additive manufacturing has recently received great interest. To increase nitrogen content in the austenitic steel powders (for example AISI 316L), two measures are taken in this study: (1) melting the steel under a nitrogen atmosphere, and (2) adding manganese to increase the solubility of nitrogen in the steel. The steel melt is then atomized by means of gas atomization (with either nitrogen or argon). The resulting powders are examined and characterized with regard to nitrogen content, particle size distribution, particle shape, microstructure, and flowability. It shows that about 0.2–0.3 mass % nitrogen can be added to the austenitic stainless steel 316L by adding manganese and melting the steel under nitrogen atmosphere. The particles are spherical in shape and very few satellite particles are observed. The steel powders show good flowability and packing density, therefore they can be successfully processed by means of laser powder bed fusion (L-PBF).

1. Introduction

Austenitic steels are non-magnetic stainless steels that contain high levels of chromium and nickel and low levels of carbon. Known for their excellent formability and corrosion resistance, austenitic steels, AISI 316L as a representative is the most widely used grade of stainless steel. Nickel is the most commonly used element to lower the austenite to ferrite/martensite transformation temperature to sub-zero temperature and stabilize the austenitic structure. However, due to the high cost and allergenic reaction on the human body, there is an increased interest in developing low nickel or nickel free austenitic stainless steels [1].

Nitrogen is also a strong austenite former. Short range ordering in the austenite lattice is enhanced by nitrogen, resulting in an increased austenite stability and solid solution strengthening because of a lower tendency to form clusters and more homogeneous distributions of substitutional atoms [2]. Since the formation of Cr atom clusters is inhibited by the addition of nitrogen, a Cr₂O₃ oxide surface layer (passive film) is formed more effectively, improving its resistance to intercrystalline corrosion, as well as the pitting and crevice corrosion [3,4,5,6]. Due to the combination of high strength, high toughness and improved corrosion resistance, high nitrogen steels (HNS) with up to 0.9 mass % N in solid solutions have become an important class of engineering materials [7,8,9]. Among them are austenitic stainless steels with low nickel and high nitrogen content, classified as AISI 200-series.

Laser additive manufacturing is an emerging technology [10,11,12,13] with unique capabilities to produce geometrically complex components that is particularly suited for small batch production, weight reduction, part-customization, and functional integration [14,15,16]. These advantages known to be relevant to high-cost alloys (superalloys or titanium alloys) and aerospace applications, are also significant for austenitic stainless steels. 316L, for example, has been satisfactorily processed into components, and their mechanical performances have allowed them to be put into service [17,18,19].

In order to further improve corrosion resistance and mechanical properties, nitrogen can be added into the austenitic stainless steels to be processed by laser additive manufacturing. One method is to melt the steel under nitrogen atmosphere, where diatomic nitrogen gas molecules dissociate and are absorbed into the melt as atomic nitrogen. However, only limited amounts of nitrogen can be added into the steel due to the low solubility of nitrogen [20]. The addition of manganese increases nitrogen solubility, allowing more nitrogen to be added [21]. Therefore, the second method used is to modify the steels by adding manganese and reducing nickel in the steels. The nitrogen content of the powder for laser additive manufacturing should not be as high as that of the HNS steels. Otherwise it may lead to the formation of gas pores and chromium nitride in the powder as well as in the additive manufactured parts.

In this study, two modified 316L stainless steels with reduced nickel content and increased manganese content have been developed based on thermodynamic calculations by our project partner at Ruhr University Bochum [22]. Nitrogen was added into the steels by melting them under a nitrogen atmosphere. The melts were then atomized into powders. The nitrogen content, particle size distribution, shape and microstructure of the particles was examined and characterized. The flowability and packing density of the powders was also determined.

2. Materials and Methods

2.1. Alloy Development

The nominal chemical composition of the base material 316L and the two modified 316L steels is listed in

Table 1. Chemical composition of the raw material 316L and modified 316L (in mass %).

Alloy	C	Cr	Ni	Mo	Mn	Si	Fe
316L	0.02	18.4	13.7	1.8	0.4	0.7	bal.
316L-Low Ni-1	0.02	18.4	11.0	1.8	6.0	0.7	bal.
316L-Low Ni-2	0.02	18.4	9.0	1.8	12.0	0.7	bal.

. The content of nickel is reduced in the two low nickel alloys to 9 mass % while manganese is increased up to 12 mass %.

An empirical formula used for the calculation of nitrogen solubility [N] in mass % in steel melts is given by [23]:

$$\left[N\right]=\sqrt{p}\times {10}^{k_s},$$

(1)

$$\begin{array}{l}{k}_s=\frac{-293}{T}-1.16-(\left(\frac{3757}{T}-0.81\right)(0.072\left[C\right]+0.051\left[Si\right]-0.015\left[Mn\right]-0.039\left[Cr\right]\\ 0.0103\left[Mo\right]+0.0093\left[Ni\right]-0.095\left[V\right]-0.0056\left[W\right]-0.059\left[Nb\right]-0.031\left[Ta\right]-0.35\left[O\right]+\\ 0.044\left[N\right])+0.5\left(\frac{5132}{T}-1.48\right)(0.0215{\left[C\right]}^2+0.000005{\left[Mn\right]}^2+0.00058{\left[Cr\right]}^2+0.00249{\left[V\right]}^2\\ +0.00068{\left[Nb\right]}^2)+0.167\left(\frac{8124}{T}-3.06\right)\left(-0.0000068{\left[Cr\right]}^3-0.00000401{\left[V\right]}^3\right)).\end{array}$$

(2)

where T is temperature in K and p is the partial pressure of nitrogen in bar (1 bar=100 kPa).

Nitrogen solubility in the austenitic stainless steels decreases with increasing melt temperature and decreasing nitrogen partial pressure (see Figure 1). The nitrogen solubility increases significantly as the manganese content is increased and the nickel content is reduced. For example, it is approximately 0.15 mass % for 316L at 0.1 MPa N₂ at a melt temperature of 1700 °C, while it is about 0.22 mass % and 0.29 mass % for the 316L-Low Ni-1 and the 316L-Low Ni-2 respectively. At 100 Pa N₂, the nitrogen solubility in the steel melts is much lower: only 50–150 ppm in the melting temperature range of the steels.

The Schaeffler diagram is a useful tool to predict the microstructure in a given composition of stainless steel [24]. The position of a steel alloy in the Schaeffler diagram is determined from its chromium and nickel equivalents. The nickel equivalent is calculated from the mass fraction of the alloying elements that cause iron-based alloys to form austenite. The chromium equivalent represents the effectiveness of the ferrite-forming elements. The Schaeffler diagram is then divided into different areas that represent different microstructures.

According to Schaeffler and Delong [25], the chromium equivalent is calculated by Equation (3):

$$C{r}_{eq}=\left[Cr\right]+1.4\left[Mo\right]+1.5\left[Si\right]+0.5\left[Nb\right]+2\left[Ti\right],$$

(3)

and the nickel equivalent is calculated by Equation (4):

$$N{i}_{eq}=\left[Ni\right]+30\times \left(\left[C\right]+\left[N\right]\right)+0.5\left[Mn\right].$$

(4)

Here the effect of nitrogen on the nickel equivalent is taken into consideration.

The position of the steel alloys 316L, 316L-Low Ni-1, and 316L-Low Ni-2 with and without the addition of nitrogen is plotted in the Schaeffler diagram (see Figure 2). The nitrogen content of the steels with added nitrogen is calculated at 0.1 MPa N₂ at the pouring temperature as listed in

Table 2. Gas atomization parameters of 316L and modified 316L steel powder.

Run Number (Exp. ID-Number)	Unit	Run 1 (PA7-159)	Run 2 (PA7-161)	Run 3 (PA7-233)	Run 4 (PA7-217)	Run 5 (PA7-234)	Run 6 (PA7-218)
Material	-	316L	316L	316L-Low Ni-1	316L-Low Ni-1	316L-Low Ni-2	316L-Low Ni-2
Atomization gas	-	N2	N2	Ar	N2	Ar	N2
Melting atmosphere	-	Ar	N	Ar	N2	Ar	N2
Delivery tube diameter	mm	2.5	2.5	2.5	2.5	2.5	2.5
Pouring temperature	°C	1650	1650	1650	1690	1650	1700
Atomization gas pressure	MPa	1.6	1.6	1.6	1.6	1.6	1.6
Atomization gas flow rate	kg/h	590	590	775	590	775	590
Melt flow rate	kg/h	-	297	268	289	268	288
GMR	-	-	1.99	2.89	2.05	2.89	2.05

. With the addition of nitrogen, the nickel equivalent is significantly increased. The 316L-Low Ni-2 shows the highest tendency to form austenite.

2.2. Experimental

2.2.1. Gas Atomization of Austenite Stainless Steel Powders

The most common process to produce metal powder for laser additive manufacturing is gas atomization: a molten metal stream is atomized by high pressure gas jets into small metal droplets, forming metal particles after rapid solidification. In this study, the gas atomization of the austenitic stainless steels was carried out on a self-constructed powder plant (PA7 at the University of Bremen) with a melting capacity of 0.7 L [26,27,28]. The main process parameters, that are selected based on our previous gas atomization experiments of steels, are listed in Table 2. The feedstock materials were inductively melted in an enclosed vessel under a nitrogen atmosphere (for the addition of nitrogen into the melt) or under an argon atmosphere (without the addition of nitrogen for reference) at an overpressure of 3 kPa. The process gas was converted from evaporated liquid nitrogen or evaporated liquid argon with a high purity of 99.99%. The steel melt flowed through a delivery tube (2.5 mm diameter) at the bottom of the crucible and was atomized by means of a self-developed close-coupled atomizer (GD26) [26]. The atomization gas pressure was 1.6 MPa, corresponding to a gas mass flow rate of 590 kg/h for nitrogen and 775 kg/h for argon. The melt flow rate was between 270 and 300 kg/h. The gas to melt mass ratio (GMR) was about 2.0 for atomization with nitrogen and 2.9 with argon. To reduce the formation of satellite particles, an additional gas jet system was applied to inhibit the circulation of the solidified particles close to the atomization zone [28]. The anti-satellite gas jet pressure was 21 kPa.

Subsequently, the gas atomized powder was mechanically sieved. The powder size fractions of <200 µm and 20–63 µm were used for testing and analysis.

2.2.2. Characterization of Austenite Stainless Steel Powders

The gas atomized powder was investigated in terms of flow properties, particle size, morphology, microstructure and chemical composition. The flowability of the powder was determined by Hall flow-testing according to ISO 4490:2018. The apparent density was measured in accordance with ISO 697:1981. Additionally, the tap density of the powder was measured using a JEL STAV II device (J. Engelsmann AG, Ludwigshafen, Germany) after 1250 tap cycles. The particle size distribution of the gas atomized powder was determined by a laser diffraction testing instrument (Malvern Mastersizer 2000, Malvern, United Kingdom). The particle shape of the powder was analyzed by a Morphologi G3 system (Malvern, United Kingdom). The nitrogen distribution in the powder was measured by an electron microprobe (JEOL JXA 8200, Peabody, USA). A carrier hot gas extraction instrument (ONH-2000 analyzer from ELTRA GmbH, Haan, Germany) was used to measure the content of nitrogen in the gas-atomized powder. Particle morphology and microstructure was investigated by means of scanning electron microscopy (CAMSCAN CS44, Cambridge, UK) and optical microscopy (Axiophot, ZEISS, Jena, Germany).

3. Results and Discussion

The cumulative mass distributions (Q3) of the particles are shown in Figure 3. The particle size distribution of the steel powders seems to be unaffected by the alloy composition and the melting atmosphere. However, in Figure 3, the particle size distribution of the 316L-Low Ni-1 (Run 4) and 316L-Low Ni-2 powder (Run 6) shifts to the left, indicating that finer particles are produced. The median diameter d50 is 39–41 µm for the two powders, while it is 46–49 µm for the other powders. A possible reason for this deviation is as follows: the pouring temperature of the melts in experiment run 4 and 6 is 40 to 50 °C higher than that of the other experiments, resulting in a lower viscosity and surface tension of the melts, and thereby producing a smaller atomized droplet size. The gas-atomized particles typically show a spherical shape and smooth surface, with a few observed satellite particles (see Figure 4).

A variety of shape parameters such as circularity, convexity, solidity, and aspect ratio are employed by the image analysis system Morphologi G3 to characterize the particle shape [29]. It is unlikely that a single shape parameter can perfectly characterize particles in all applications. For example, an application concerned with perfectly spherical particles could use circularity to measure any deviation. However, circularity would not be appropriate in an application where both ‘spikey’ and elliptical particles are present in the sample. Since the gas atomized particles are generally in spherical shape, only circularity of the steel powders is presented in this paper. According to the definition by the Morphologi G3 [29], Circularity is the ratio of the circumference of the projected/equivalent area of a particle to the actual perimeter of the particle:

$$\mathrm{Circularity}=\frac{2\sqrt{\pi ·\mathrm{Area}}}{\mathrm{Perimeter}}$$

(5)

Circularity quantifies how close the shape is to a perfect circle. A perfect circle has circularity of 1.0, while a very narrow, elongated particle has circularity close to 0.

The shape parameter circularity of the various steel powder particles is presented in Figure 5. The circularity of the powder particles (size 20–63 µm) is in a range of 0.93 to 0.97, with a higher circularity for the smaller particles. For the large particles with a size of 200 µm the circularity is about 0.90. The small particles are cooled and solidified faster than the large particles, and it is less possible for the collided particles to be embedded in the small particles that are completely solidified. The modified 316L powders show a higher level of circularity than the 316L powders, possibly due to lower viscosity and higher surface tension of the melts. In general, a high circularity means good flowability of the powder. The spherical particles have less contact surface area, therefore the friction and interlocking between the particles is lower than that of the particles in irregular shape [28,30].

The nitrogen content in the feedstock (316L and modified 316L prepared by inductive melting and casting) is 200–300 ppm. It is increased in the atomized particles (see Figure 6). When melting under an argon atmosphere, the nitrogen content is 600–800 ppm, independent of the chemical composition of the stainless steels. Under a nitrogen atmosphere, the nitrogen pick-up is significantly increased. The addition of manganese in the austenitic stainless steel also contributes significantly to the pick-up of nitrogen in the steel.

The measured nitrogen content in the powders is very close to the calculated values as shown in Section 2.1. This means that almost all of the dissolved nitrogen in the steel melts remains in the atomized powders. The solidification of the steel melts possibly with primary δ-ferrite, which has a very low solubility for nitrogen [20], did not result in significant nitrogen outgassing due to rapid solidification of the atomized droplets.

After mechanical sieving, the powder fraction of 20–63 µm is investigated further. This powder fraction accounts for approximately 45 percent of the total atomized powder. The flow time of this powder is 15–16 s for a 50 g powder sample through a Hall flow funnel (outlet diameter 2.5 mm). The good flowability of the powder is attributed to the spherical shape of the particles and very few satellite particles. These satellite particles are formed when the finer solidified particles stick to the molten or semi-molten surface of the coarser ones as a result of the in-flight collisions. There are very few satellites in the powder due to the use of the anti-satellite system in the gas atomization. The apparent density and the tap density of the powder is 4.5 g/cm³ and 5.0 g/cm³, respectively. These powder particles have been successfully processed by means of laser powder bed fusion, which will be reported in the future.

The particle morphology of the gas-atomized powder is shown in Figure 7. Typical dendritic structure can be observed at the particle surface. It is also clearly seen in the metallographic sections of the particles processed under a nitrogen atmosphere (see Figure 8). However, the dendrites are not so apparent in the particles processed under argon melting atmosphere. It appears that the chemical etchant used erodes the dendrites more significantly in the interior region than at the boundaries of the dendrites. This difference between the etching effects correlates to the different nitrogen concentrations in the powders. The addition of nitrogen to the stainless steel promotes a more homogeneous distribution of chromium in the matrix, and therefore an improved corrosion resistance [2,7,8]. According to the Schaeffler diagram, the 316L and the modified 316L alloys are composed of austenite and small fraction of ferrite (less than 5%). With addition of nitrogen, the alloys are essentially austenite. From the micrographs it is difficult to distinguish the ferrite from the austenite.

Element mapping (EPMA) on the cross section of the particles show that the nitrogen distribution in the particles is relatively homogeneous. Between the outer surface and the core of the particles, no nitrogen concentration gradient is observed, as represented in Figure 9. The average nitrogen concentration of the 316L-Low Ni-2 powder particles processed under a nitrogen atmosphere is about 0.3 mass %, which is in good agreement with the measured results in Figure 6. Slight segregation of chromium is seen by element mapping, while the segregation of the alloying elements manganese, nickel, and molybdenum in the dendrite structure of the particle is more significant. Since the segregation occurs in micrometer scale, it could have a very limited effect on the laser additive manufactured parts.

4. Conclusions

Two modified 316L steel alloy powders are developed by reducing nickel and increasing manganese in the steels. The nitrogen solubility in the modified steels is increased to 0.2–0.3 mass % at 0.1 MPa N₂ and a melt temperature of 1650–1700 °C. The addition of manganese and nitrogen in the steel leads to higher stability of austenite.

The modified 316L steel alloys dissolve nitrogen to the solubility limit during melting; a relatively high amount of nitrogen is kept after atomization with the process gas nitrogen. Depending on the alloy composition, about 0.2–0.3 mass % nitrogen can be achieved in the gas-atomized powders. Due to the rapid solidification of the atomized particles, nitrogen released during solidification is not significant. Nitrogen is homogeneously distributed from the periphery to the center of the particles.

The particles show spherical shape and very few satellite particles. They have good flowability and packing density, meaning that they can be satisfactorily processed by laser powder bed fusion (L-PBF). The solidification microstructure of the powder particles is very fine, and it is typically dendritic. According to the Schaeffler diagram, the 316L and the modified 316L alloys are composed of austenite and small fraction of ferrite (less than 5%). With addition of nitrogen, the alloys are essentially austenite.