Heat treatment is always a major topic at Powder Metallurgy conferences since sintered components for applications under high loading usually require a heat treatment. Oral presentations at Euro PM2013 in Gothenburg covered innovative heat treatments that minimise dimensional change such as low pressure carburising and nitrocarburising. In these contributions the benefits of prealloyed steel powders containing chromium and/or molybdenum were prominent. A comprehensive overview of steam treatment, including a sophisticated experimental procedure and design options for better performance of PM gears, completed Session 17.
Magnus Dahlström, Höganäs AB, Sweden, investigated the effects of low pressure carburising and subsequent high pressure gas quenching on the properties of prealloyed molybdenum and chromium containing low alloy PM steels. The outstanding benefit of this process over traditional gas carburising is that it is oxygen-free and therefore chromium steels can be carburised without oxidation.
The distortion caused by gas quenching is milder and therefore less than by oil quenching. However, this calls for the use of steels with a better hardenability than simple Fe-Cu alloys. Further, the process is regarded as more energy efficient, causing less CO2 emissions and avoiding the need for the cleaning of parts after case hardening.
Dahlström determined the microstructures and hardness profiles of PM steels based on Astaloy 85Mo, Astaloy CrA and Astaloy CrL powders and provided evidence of fully martensitic structures in the hardened zone and bainite/martensite structures in the core. An example is shown in Fig. 1.
Unique opportunities for PM technology in optimised gear design were demonstrated by Michael Andersson, Höganäs AB. While the shape of machined gears is limited by the evolving motion of the machining tool, dies for PM technology offers innovative design opportunities for gears that reduce local stresses in the tooth root.
Finite element analysis was used to determine the maximum stresses in the gear and then the radius of curvature in the tooth root was reduced and at the same time the material thickness was increased. Relatively minor modifications that are hardly visible with the naked eye were sufficient to reduce the maximum stresses by up to 30%. The author suggested that the optimisation of the tooth root geometry should be a routine step in PM gear design.
Gian Filippo Bocchini, PM Consultant, Rapallo, Italy, gave a comprehensive overview of the effects of steam treatment on the mass gain depending on density, geometry and process conditions. In particular he investigated the local mass gain after steam treatment in a sophisticated experimental procedure. The local mass gain was determined on cylindrical steam treated samples by turning off thin slices on the outer diameter and the end faces.
The weight of the removed material in relation to the removed volume, i.e. the local density, was set in relation to the original density before steam treatment. In a plot of the mass gain versus the square-root of the distance from the surface for three different densities he found straight lines (Fig. 2) and thus demonstrated that for thick parts a square-root law holds.
Belt pulleys and the severe wear conditions in cars when driving on dusty roads were taken by Caroline Larsson , Höganäs AB, as an example of the need for nitrocarburising heat treatment. A comparison was presented of a Fe-2% Cu-0.5% C alloy blend and prealloyed Astaloy CrA + 0.35% C both sintered at 1120°C/30 min in a 90% N2/10% H2 atmosphere. The best results were achieved with a steam treatment at 510°C/90 min followed by nitrocarburising at 580°C/100 min.
The wear properties of nitrocarburised surfaces of PM steels were evaluated with the dry sand-rubber wheel test according to ASTM G65. In this standardised test the sample is pressed against a rotating rubber wheel with a certain force and standardised sand pouring out of a hopper is the abrasive medium. The wear properties were compared in the as-sintered, steam treated and steam treated + nitrocarburised condition.
As expected, the chromium containing steel performed clearly better in the wear test since chromium forms hard nitrides in a well-defined compound layer and in the diffusion zone. A further improvement was achieved for the sample with MnS addition that also showed a higher C-content (Fig. 3).
Author: Dr Georg Schlieper, Gammatec Engineering GmbH, Germany
Dr.-Ing. Georg Schlieper, physicist, received his PhD at the Insitute for Materials and Solid State Research of the University of Karlsruhe, Germany. He worked for 15 years in product and process development for the Powder Metallurgy industry where he focused on high strength sintered steels, heat treatment, surface technology, magnetic materials and metal injection moulding. Since 1994 he has worked independently as a consultant. Email: [email protected]