Microstructure characteristics of spray-formed high speed steel and its evolution during subsequent hot deformation
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Yong-an Zhang State Key Laboratory of Non-Ferrous Metals and Process, General Research Institute for Non-Ferrous Metals, Beijing 100088, China
Ji-Shan Zhang State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China (Received 8 September 2015; accepted 21 December 2015)
The microstructural evolution of spray-formed high speed steel during hot deformation was investigated as well as the effects of spray forming parameters on the porosity formation. Four distinct zones are identified in the as-deposited material, and interstitial porosity is present in the bottom and peripheral zones, while gas-related porosity is mainly found in the central zone. It can keep the porosity at a minimum value by using the optimum parameters, e.g., the average porosity of central zone is 3.7% for a superheat of 170 °C and a gas-to-metal flow rate of 0.7. During hot deformation at 1150 °C, the amount of porosity can be obviously decreased by increasing the height reduction which also plays a key role in breaking up eutectic carbides. The critical height reduction for the breakdown of the eutectic carbides is 50%, the dominant mechanism being mechanical fragmentation.
I. INTRODUCTION
Spray forming, which was invented by Prof. Singer in the early 80s, has been proved to be an effective method to improve the solidification microstructure of tool steels1–6 due to the effects of high cooling rate (103;105 K/s) during atomization and deformation or fragmentation during deposition.7,8 It is also widely accepted that the advantages of spray-formed materials are the very fine scale microstructures, fine precipitates, low levels of segregation, extended solid solubility, and presence of metastable phases. One of the main disadvantages of spray-formed materials is the presence of porosity. Although porosity is not always deleterious in spray-formed materials, it is generally undesirable, especially for applications where strength and ductility are critical. There are lots of studies about the porosity formation9–12 and the effects of processing parameters on its formation.13–15 According to these proposed mechanisms of the porosity formation, we could divide the porosity into three categories, such as gas pores, interstitial pores and solidification shrinkage. The gas pores are related to the
Contributing Editor: Yang-T Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.407 274
J. Mater. Res., Vol. 31, No. 2, Jan 28, 2016
presence of an excessive proportion of liquids during deposition. It is reported that for the same material, the gas pores are relatively high when the liquid fraction is high.11,16 The solidification shrinkage porosity is produced as a result of the large differences in density between the liquid and solid phases.11 Some studies17,18 show that a large portion of porosity observed in sprayformed materials may be attributed to interstitial pores. This type of porosity is generally a result of the incomplete filling o
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