Steels for bearings

The “quenching and partitioning” (Q&P) process has been studied in a low-carbon steel containing 1.1 wt pct aluminum by heat treatments consisting of partial austenitization at 900 °C and subsequent rapid cooling to a quenching temperature in the range between 125 °C and 175 °C, followed by an isothermal treatment (partitioning step) at 250 °C and 350 °C for different times. Characterization by means of optical and scanning electron microscopy, electron backscattered diffraction (EBSD), magnetization measurements, and X-ray diffraction (XRD) has shown a multiphase microstructure formed by intercritical ferrite, epitaxial ferrite, retained austenite, bainite, and martensite after different stages of tempering. A considerable amount of retained austenite has been obtained in the specimens partitioned at 350 °C for 100 seconds. Experimental results have been interpreted based on concepts of the martensite tempering, bainite transformation, and kinetics calculations of the carbon partitioning from martensite to austenite.

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Microstructural Evolution of a Low-Carbon Steel during Application of Quenching and Partitioning Heat Treatments after Partial Austenitization

Effect of rare earth cerium addition on the microstructure and tensile properties of hypereutectic Al–20%Si alloy

Characterization of the microstructure obtained by the quenching and partitioning process in a low-carbon steel

Wear behavior and microstructure of hypereutectic Al-Si alloys prepared by selective laser melting

Spray forming of hypereutectic Al–Si alloys

In order to minimize shape deviations in hardening of gear wheels, all production steps in the manufacture of disks made of SAE 5120 prior to heat-treatment were analyzed by means of design of experiments (DoE). The influence of hardenability, pre-heat treatment, forging temperature, and feed rate and partition of material removal in cutting on the distortion of disks were investigated. Standard case hardening of the disks led to a characteristic shape change called “dishing”. The statistical analysis of the test results has proven that dishing of disks is mainly affected by the partition of the material removal in the considered case. In additional experiments dishing could be eliminated (on average) by an optimized partition of the material removal. However, results from experiments with spray-formed disks and with disks using a different forming strategy imply that the material flow combined with material inhomogeneities are the physical causes for dishing rather than the partition of material removal.

Distortion minimization of disks for gear manufacture

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

The deformation behavior of spray-formed hypereutectic aluminum–silicon alloys—AlSix (x = 18, 25, and 35 wt%)—has been studied by means of compression test at various temperatures and strain rates. The flow stress of the spray-formed Al–Si alloys increases with decreasing compression temperature and increasing strain rate. Higher silicon content in the alloys also leads to higher flow stress during deformation. The flow curves determined from the compression tests exhibit that the deformation of the materials is controlled by two competing mechanisms: strain hardening, and flow softening. Particle damage during the deformation may have an influence on the flow curves of the alloys with large silicon particles. Based on the flow curves obtained from the compression tests and knowledge of aluminum extrusion, the spray-formed hypereutectic Al–Si alloy billets have been hot extruded into wires with a high area reduction ratio around 189. Since primary silicon particles were greatly refined and uniformly distributed in the spray-formed materials, the heavy deformations of the spray-formed Al–Si alloys containing high amount of silicon were successfully performed.

Chengsong Cui

Papers

Spray forming of hypereutectic Al–Si alloys

Distortion minimization of disks for gear manufacture

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

Deformation behavior of spray-formed hypereutectic Al–Si alloys