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Progress in Materials Science Vol. 9. Edited by Dr. Bruce Chalmers. (Incorporating “Progress in Metal Physics”, Volumes 1–8.) Pp. 389. (London and New York: Pergamon Press, 1961.) 120s. net; 20 dollars.

Progress in Materials Science

Heat Treatment of metals is the controlled heating and cooling of metals to optimize their physical and mechanical properties for use in the oral environment. Heat treatment is primarily of two types i.e. Softening Heat Treatment and Hardening Heat Treatment. The softening heat treatment is given to reduce strength and hardness, remove residual stresses, restore ductility after cold working procedures and refine grain size to improve life and appliance performance. Hardening Heat Treatment is given to the metal to increase strength and wear properties during actual working conditions. In this brief review it is attempted to highlight the basic mechanisms involved during heat treatment of metals and the methods of heat treating metals in actual practice.

Heat treatment of metals

Microstructure evolution of railway pearlitic wheel steels under rolling-sliding contact loading

Cryogenic treatments are considered a good way to reduce the retained austenite content and improve the performance of tool steels. Four different heat treatments, two of which included a deep cryogenic stage, were applied in this study to an H13 tool steel, subsequently determining its mechanical properties by means of tensile, hardness and fracture toughness tests. Furthermore, scanning electron microscopy and X-ray diffraction analysis were performed to gain an insight into the microstructural evolution of these heat treatments during all the stages. It was concluded that the application of a deep cryogenic treatment to H13 steel induces higher thermal stresses and structural defects, producing a dispersed network of fine carbides after the subsequent tempering stages, which were responsible for a significant improvement in the fracture toughness of this steel without modifying other mechanical properties. Although the application of a deep cryogenic treatment reduces the retained austenite content, there is a minimum innate content which cannot be transformed by heat treatment. Nevertheless, this austenite is hence believed to be stable enough and should not transform during the normal service life of forging dies.

The effect of deep cryogenic treatments on the mechanical properties of an AISI H13 steel

Wear behavior of bainitic and pearlitic microstructures from microalloyed railway wheel steel

Recent literature has shown several papers dealing with cryogenic treatments of tool steels. Most of them see an improvement in wear resistance supposedly due to microstructural modifications that occur at cryogenic temperatures. Cryogenic treatments are supposed to modify the way secondary carbides precipitate, obtaining a finer and more homogeneous distribution. Specimens of an AISI D2 tool steel were submitted to different thermal cycles, including cryogenic treatment, and the volumetric fraction of micrometric carbides was determined. The samples were analyzed by scanning electron microscopy and the image analysis was carried out with Image J software. The present work also discusses what is understood about secondary carbides and the modifications that occur with micrometric (from the austenite-carbide field) and nanometric (from the temper of martensite) secondary carbides. No difference was found between the micrometric carbide with and without cryogenic treatment, while some indications were found that the nanometric carbides are refined and more homogeneously distributed after cryogenic treatment.

Microstructural Characterization of an AISI D2 Tool Steel Submitted to Cryogenic Treatment

The Heavy-Haul railroad wheels started to use higher wear resistance steels microalloyed with niobium, vanadium, and molybdenum [1]. During continuous cooling, these elements depress the temperature of the pearlite formation, producing smaller interlamellar spacing that increases the hardness of the steel, besides to favor the precipitation hardening through the formation of carbides [2, 3]. Also, they delay the formation of difusional components like pearlite and bainite during isothermal transformation. The effects of these alloy elements on microstructure during isothermal transformation were studied in this work using a Bähr 805A/D dilatometer. Three different compositions of class C railway wheels steels (two microalloyed and one, non microalloyed) were analyzed in temperatures between 200 and 700 °C. The microstructure and hardness for each isothermal treatment were obtained after the experiments. Comparing with non microalloyed steel (7C), the vanadium addition (7V steel) did not affect the beginning of diffusion-controlled reactions (pearlite and bainite), but delayed the end of these reactions, and showed separated bays for pearlite and bainite. The Nb + Mo addition delayed the beginning and the ending of pearlite and bainite formation and also showed distinct bays for them. The delays in diffusion-controlled reactions were more intense in the 7NbMo steel than in 7V steel. The V or Nb + Mo additions decreased the start temperature for martensite formation and increased the start temperature for austenite formation.

Effect of Alloy Elements in Time Temperature Transformation Diagrams of Railway Wheels

Effect of niobium and molybdenum addition on the wear resistance and the rolling contact fatigue of railway wheels

The use of magnetic Barkhausen (MBN) noise methods to detect the austenite to martensite phase transformation during cooling of steel specimens was explored, using three different configurations: conventional Barkhausen noise emission stimulated by an alternating magnetic field, the Okamura method, that measures magnetic noise emission under a fixed DC field and a novel experimental technique that measures spontaneous magnetic emission during transformation, in the absence of any external field. The phenomena associated with the phase transformation were also followed by electrical resistivity measurements and the resulting samples were characterized using optical and scanning electron microscopy. MBN measurements on a AISI D2 tool steel austenitized at 1473K and cooled to liquid nitrogen temperature exhibited a clear change near 225 K during cooling, corresponding to the Ms (martensite start ) temperature, as confirmed by resistivity measurements. Spontaneous magnetic noise emission measurements made in situ during cooling of a sample immerged in liquid nitrogen showed that individual “burst” phenomena could be detected, in a manner similar to acoustic emission (AE) measurements. This magnetic emission (ME) can thus be considered a new experimental tool for the study of martensite transformations in ferrous alloys.

Paula Fernanda da Silva Farina

Papers

Wear behavior of bainitic and pearlitic microstructures from microalloyed railway wheel steel

Microstructural Characterization of an AISI D2 Tool Steel Submitted to Cryogenic Treatment

Effect of Alloy Elements in Time Temperature Transformation Diagrams of Railway Wheels

Effect of niobium and molybdenum addition on the wear resistance and the rolling contact fatigue of railway wheels

The Use of Barkhausen Noise to Study the Martensitic Phase Transformation