19 April, 2017: Runcorn: LPW Technology Ltd, the market leader in the development, processing and supply of high quality metal powders and solutions for the Additive Manufacturing (AM) industry, has added its latest case study to the fast-growing suite of technical materials on its website.
‘Nitrogen vs Argon Atomisation of 17-4 PH Stainless Steel and its Effects on AM Processing’ joins the current case studies in the LPW technical library, alongside its brochures and helpful powder finding tool.
“It’s important to understand the effect that powder production methods and AM processing parameters can have on the resulting component microstructure,” says Dr Rob Deffley, LPW’s Research & Development Manager. “LPW’s highly-trained team understands the behaviour of powders processed under different conditions, and can recommend the appropriate powder to achieve the required mechanical properties. This latest case study addresses a question we have been asked by several AM end users, and adds further intelligence to our portfolio of technical information.”
LPW has state-of-the-art laboratory facilities and undertakes a wide range of research and development. To view LPW’s most recent case study, please press here or read below.
Nitrogen vs. Argon Atomisation of 17-4 PH Stainless Steel and its Effects on AM Processing
Background on 17-4 PH (Stainless Steel)
17-4 PH is a martensitic, precipitation-hardenable stainless steel widely used in aerospace, petroleum, and chemical processing industries. This 17-4 PH alloy utilises a martensitic microstructure infiltrated with fine particles to optimise the mechanical properties. It contains large amounts of chromium, nickel and copper to generate good corrosion resistance and mechanical properties at elevated temperatures (up to 300°C).
This material is heat-treatable to increase the hardness, ultimate tensile strength, and yield strength. The typical heat-treatment cycle of 17-4 PH begins with a high temperature austenitisation treatment (1040°C for 3 minutes) to allow super saturation of alloying elements into the matrix. This material is then air cooled to form a martensitic microstructure. Fine, copper-rich particles are precipitated by an age-hardening step (482°C for 1 hour) to improve the alloy’s strength.
Why is the atomisation process important?
The choice of atomising gas on the properties of metal powders, in conjunction with the specific additive manufacturing process parameters, plays a key role in the ultimate microstructure and mechanical properties of fabricated parts as demonstrated in this Selective Laser Melting (SLM) study. Using an inappropriate combination of powder and process can result in the expected final properties of a component not being fully realised.
Effect of selection of powder atomisation gas and AM process parameters on 17-4 PH Stainless Steel
17-4 PH can be atomised using argon or nitrogen, however, the resulting microstructure of 17-4 PH must be martensitic to deliver the optimum mechanical properties.
Atomising 17-4 PH with argon will deliver a powder with the desired martensitic microstructure, whereas nitrogen gas atomisation will deliver an austenitic microstructure. Whether the powder is atomised using argon or nitrogen, subsequent processing of the powder with an argon SLM processing gas will still deliver a martensitic part that can be heat treated to produce enhanced mechanical properties. However, using argon for both atomisation and processing will deliver optimum hardness, ultimate tensile strength, and yield strength values. Using nitrogen for both processes will result in the austenitic microstructure with no observed increase in desired mechanical properties.
17-4 PH powders were created using two different atomisation gases, nitrogen and argon . The chemistries of both powders were within the 17-4 PH specification. Preliminary powder cross sections of the argon atomised powder revealed a martensitic microstructure while the nitrogen atomised powder was austenitic.
Four total builds were completed to study the effect of powder atomisation process and machine environment on the resultant microstructure as shown in Table 1.
All builds that used argon as either the atomisation or SLM processing gas had a martensitic microstructure. The hardness data for these four materials are shown in Table 2. The hardness of the builds with a martensitic structure increased to over 400 HV following heat treatment. However, the build that used nitrogen as the atomisation and processing gas contained a large volume fraction of retained austenite that had no change in hardness.
Nitrogen is a small, interstitial element that, in low concentrations, is soluble in the face centred cubic (FCC) austenite matrix that is normally observed at high temperature . A study by Biggs and Knutsen  suggests that steels with a higher concentration of nitrogen have a higher stacking fault energy which inhibits the nucleation of martensite. This shows that increasing nitrogen concentration, even by small amounts, will reduce the amount of martensite at room temperature and thus affect the heat treatability of the material.
Therefore, when the powder is atomized in a nitrogen atmosphere, and then processed within a nitrogen atmosphere, it is plausible that the nitrogen content increases to the point at which it starts to inhibit the martensite formation.
Although the two feedstock materials had only small differences in chemistry, the change in powder production processing route and SLM fabrication conditions resulted in significant differences in the final mechanical properties. This was attributed to different microstructures obtained using different processing routes.
It is clearly important to understand the effect that powder production methods and AM processing parameters can have on the resulting component microstructure.
LPW’s highly-trained team understands the behaviour of powders processed under different conditions and can recommend the appropriate powder to achieve the required mechanical properties. LPW can supply both nitrogen and argon atomised 17-4 PH powders.
 L. E. Murr et al., “Microstructures and properties of 17-4 PH stainless steel fabricated by selective laser melting,” J. Mater. Res. Technol., vol. 1, no. 3, pp. 167–177, 2012.
 G. Krauss, Steels: Processing, Structure, and Performance. Materials Park, OH: ASM International, 2005.  T. Biggs and R. D. Knutsen, “The effect of nitrogen on martensite formation in a Cr-Mn-Ni stainless steel,” J. Phys. IV, vol. 5, pp. 515–520, 1995.
Established in 2007, LPW Technology is the market leader in the development, processing and supply of metal powders for additive manufacturing, and provides a comprehensive range of services for the AM industry. These services include the development of new alloys and expert application support. The company has developed a full range of optimised powders specifically for Selective Laser Melting (SLM), Laser Metal Deposition (LMD) and Electron Beam Melting (EBM) with standard powders supplied from stock, and custom and development alloys available on request.
LPW Technology has developed PowderLife®, a unique AM powder lifecycle management system which strictly controls risk for manufacturers, adding confidence, reliability and traceability in metal powder production and repeated AM builds. LPW invests heavily in cutting edge analytical technology and offers a complete powder analysis service.
LPW Technology has its headquarters in Runcorn, Cheshire, UK. In June, 2014 the company formally established a US subsidiary LPW Technology Inc. situated in Pittsburgh, Pennsylvania, providing analytical services, product inventory and sales support to North and South America. From these locations the company supplies high quality, certified powders to a global customer base including aerospace, biomedical and automotive industries. In 2015, it opened its German sales office and has a global network of resellers in China, Israel, Italy, Japan, Korea, Russia, Singapore and Turkey.
The company operates to quality control standards: AS 9100 & AS 9120 for aerospace, ISO 9001, and ISO 13485 for medical.
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LPW Technology is the proud recipient of the Queen’s Award for Enterprise in International Trade 2016.