Iron and steel powder grades are extensively used in the powder metallurgy (PM) industry for manufacturing of ferrous structural components, with a major customer in the automotive industry. The powder grades are predominantly manufactured by water-atomization, yielding powder particles with highly irregular morphology and good compressibility, making the powder ideal for the conventional press-and-sinter manufacturing route. The particles of the as-received powder are covered by a surface oxide layer, about 5-7 nm thick, as a result of the exposure to air or oxygen-containing atmospheres. The oxide layer is readily removed at low temperatures in the range 250-500 °C using hydrogen as a reducing agent, but the reduction process is strongly affected by the initial state of the powder and the alloy composition, as well as several processing parameters such as prior compaction of the powder and the composition of the processing gas. Since it contains a large fraction of the total oxygen content in the powder, removal of the oxide layer is a crucial step in the processing of these powder grades. Further raising the temperature enables the progressive reduction of various oxides as reflected by their thermodynamic stability, with oxides containing Cr, Mn and Si requiring significantly higher reduction temperatures than iron-based oxides. The progress of oxide reduction was studied using thermogravimetric and kinetic analysis and complemented with surface analysis methods. Parallel to the chemical processes, shrinkage occurs as the powder particles are sintered. This increases the density of compacted powder as particles are bonded to each other and porosity decreases. The dimensional changes during sintering, as observed by dilatometry, are shown to be a function of compaction pressure, the direction relative to the compaction direction as well as multiple processing parameters such as the processing atmosphere, heating rate and sintering temperature where a significant phase-dependence exists. A large fraction of the total sintering shrinkage is attained already at low temperatures in the ferrite phase of the iron-based matrix. The interaction between the chemical and physical processes are shown to be important for sintering as oxide reduction and oxygen removal are strongly dependent on the initial density of the compacted powder. Overall, oxide reduction is seen as a major goal in modern PM steelmaking in order to facilitate the development of strong, defect-free sinter necks. At the same time, sintering shrinkage and anisotropy must be considered as well to ensure dimensional tolerances. These two phenomena occur simultaneously, with large magnitudes already at temperatures significantly lower than standard sintering temperatures, indicating that the heating stage of the sintering process requires careful control in order to successfully sinter water-atomized iron and steel powder grades.

中文翻译：

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水雾化铁粉和低合金钢粉烧结过程中表面化学性质的演变

钢铁粉末等级广泛用于粉末冶金 (PM) 行业，用于制造黑色结构部件，主要客户是汽车行业。粉末等级主要通过水雾化制造，产生具有高度不规则形态和良好压缩性的粉末颗粒，使粉末成为传统压制和烧结制造路线的理想选择。由于暴露在空气或含氧气氛中，原样粉末的颗粒被表面氧化层覆盖，约 5-7 nm 厚。使用氢气作为还原剂，在 250-500 °C 的低温下很容易去除氧化层，但还原过程受粉末的初始状态和合金成分的影响很大，以及一些加工参数，例如粉末的预先压实和加工气体的成分。由于它包含粉末中总氧含量的很大一部分，因此去除氧化层是这些粉末等级加工中的关键步骤。进一步升高温度可以使各种氧化物逐渐还原，正如它们的热力学稳定性所反映的那样，含有 Cr、Mn 和 Si 的氧化物需要比铁基氧化物显着更高的还原温度。使用热重分析和动力学分析并辅以表面分析方法研究了氧化物还原的进展。与化学过程平行，当粉末颗粒被烧结时会发生收缩。随着颗粒相互结合和孔隙率降低，这会增加压实粉末的密度。通过膨胀测定法观察到的烧结过程中的尺寸变化显示为压实压力、相对于压实方向的方向以及多个加工参数（例如加工气氛、加热速率和烧结温度）的函数，其中显着相-存在依赖。铁基基体的铁素体相在低温下已经达到了总烧结收缩的很大一部分。化学和物理过程之间的相互作用对烧结很重要，因为氧化物还原和氧去除强烈依赖于压实粉末的初始密度。全面的，减少氧化物被视为现代粉末冶金炼钢的主要目标，以促进坚固、无缺陷的烧结颈的发展。同时，还必须考虑烧结收缩和各向异性，以确保尺寸公差。这两种现象同时发生，并且在显着低于标准烧结温度的温度下已经发生了很大的变化，这表明烧结过程的加热阶段需要仔细控制才能成功烧结水雾化钢铁粉末等级。