Steels for bearings

Laser melting (LM), with a focused Nd: YAG laser beam, was used to form solid bodies from a 316L austenite stainless steel powder. The microstructure, phase content and texture of the LM stainless steel were characterized and compared with conventional 316L stainless steel. The crack-free LM samples achieved a relative density of 98.6±0.1%. The XRD pattern revealed a single phase Austenite with preferential crystallite growth along the (100) plane and an orientation degree of 0.84 on the building surface. A fine columnar sub-grain structure of size 0.5 μm was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10–100 μm. Molybdenum was found to be enriched at the sub-grain boundaries accompanied with high dislocation concentrations. It was proposed that such a sub-grain structure is formed by the compositional fluctuation due to the slow kinetics of homogeneous alloying of large Mo atoms during rapid solidification. The local enrichment of misplaced Mo in the Austenite lattice induced a network of dislocation tangling, which would retard or even block the migration of newly formed dislocations under indentation force, turning otherwise a soft Austenite to hardened steel. In addition, local formation of spherical nano-inclusions of an amorphous chromium-containing silicate was observed. The origin and the implications of the formation of such oxide nano-inclusions were discussed.

Hardened austenite steel with columnar sub-grain structure formed by laser melting

Laser based additive manufacturing in industry and academia

Investigations on the microstructure and crack formation of IN738LC samples processed by selective laser melting using Gaussian and doughnut profiles

Transformation of austenite to duplex austenite-ferrite assembly in annealed stainless steel 316L consolidated by laser melting

Spray forming of hypereutectic Al–Si alloys

Microstructure and Properties of Selective Laser Melted High Hardness Tool Steel

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).

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

Bulk synthesis by spray forming of Al–Cu–Fe and Al–Cu–Fe–Sn alloys containing a quasicrystalline phase

Spray forming has proven to be a useful tool for generating high-alloyed materials. The metallurgical advantages of high solidification rates are obvious. Shortening of process chains in comparison to powder metallurgy is often discussed. To approach to some of the remaining questions, especially on the influence of process gases on spray forming steels, within this study high-alloyed steels with melt weights up to 150 kg are spray formed and hot worked to semi-finished materials. Properties are analyzed in comparison to products available on the market. Cold-work tool steels with high carbon content can be spray formed to produce materials with good wear resistance and toughness. When reducing the carbon content and the amount of hard carbides, advantage of reduced segregation becomes more apparent. On the other hand, the influence of nitrogen as an alloying element is more effective and means of control are required. When corrosion or heat resistant steels with high non-carbide bonded chromium content or other nitride forming elements are part of the alloy, care has to be taken on controlling the dissolution of process gases and subsequent precipitation and degassing during compaction and further processing. Experiments where melt protection and atomization gas were changed from nitrogen to argon have shown significant influence of process gases on spray forming results and the mechanical properties of the materials in hot worked and finally heat treated condition.

Alwin Schulz

Papers

Spray forming of hypereutectic Al–Si alloys

Microstructure and Properties of Selective Laser Melted High Hardness Tool Steel

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

Bulk synthesis by spray forming of Al–Cu–Fe and Al–Cu–Fe–Sn alloys containing a quasicrystalline phase

Opportunities and challenges of spray forming high-alloyed steels