Nitriding of high speed steel

Abstract

Current practice when nitriding high speed steel (HSS) cutting tools is to avoid embrittlement of the cutting edge by limiting the depth of the diffusion zone. This is accomplished by reducing the nitriding time and temperature and eliminating any compound layer formation. However, in many applications there is an argument for generating a compound layer with beneficial tribological properties. In this investigation, results are presented of a metallographic, X-ray diffraction and X-ray photoelectron spectroscopy analysis of nitrided HSS surface layers generated using active screen plasma nitriding and reactive vapour deposition using cathodic arc. These results are discussed in the context HSS cutting tool performance when machining under built-up edge conditions.     

The effect of nitrogen on the cold forging properties of 1020 steel

We have studied the effect of nitrogen on the cold forging properties of a low carbon steel as a function of temperature. Five AISI 1020 steels with nitrogen contents from 12 to 180 ppm were examined by tensile testing from 25 to 371 °C. Yield strength, tensile elongation (ductility), ultimate tensile strength (UTS), strain hardening exponents and strength coefficients were determined. The influence of nitrogen on the mechanical property behavior of this low carbon steel exhibits trends as expected—when nitrogen content increases, the strength of the steel increases and the ductility decreases. Likewise, as the temperature increases, the strength of the steel generally decreases; however, the ductility initially decreases, then exhibits an increasing trend. Additionally, there is an intermediate temperature range for these alloys where anomalous behavior is observed. Serrated stress–strain curves seen in this temperature range are indicative of dynamic strain aging. It is probable that this anomalous mechanical property trend is due to dynamic strain aging.     

Sliding wear behavior of salt bath nitrocarburized medium carbon steel

Salt bath nitrocarburizing is a well-known thermochemical diffusion process for enhancing the tribological and corrosion properties of ferrous components. The current work describes the role of a compound layer developed during nitrocarburizing, both in the ferritic and austenitic regimes of Fe-N-C system, on the sliding wear behavior of a medium carbon steel. The wear behavior of the nitrocarburized steel discs was assessed by the pin-on-disc tests (ASTM G 99-99) under different normal loads running against a hardened SAE52100 pin. It was observed that the compound layer on the surface not only controlled the wear rate but also resisted the adhesive wear/transfer of material from pin to disc, aside from providing low-friction coefficients.     

Effect of strain rate and temperature on the tensile properties of MANET II steel

MANET II, a modified 12% Cr steel with the German designation DIN 1.4914, is a candidate structural material for the first wall and blanket in fusion reactors. In the present study, the tensile properties of this steel were investigated in the temperature range of 25 to 350 °C at strain rates of 5 x 10^-5 ,1.2 x 10^-4, and 1.2 x 10^-3s-1, Both microstructure and fracture surfaces were examined using optical and scanning electron microscopic (SEM) techniques. The results showed that the steel suffers dynamic strain aging, although no serrated flow was observed. Yield strength, ultimate strength, and elongation showed negative strain rate sensitivity. Dynamic strain aging also affected the strain hardening rate. Results are discussed with regard to the chemical composition and fracture surface morphology.     

304奥氏体不锈钢低温盐浴渗氮处理

采用430℃低温盐浴对304奥氏体不锈钢进行渗氮处理,研究了渗氮时间对渗氮层组织和性能的影响。利用XRD衍射仪、光学显微镜、表面显微硬度计和带能谱仪(EDS)的扫描电镜(SEM)分别分析渗氮层的相组成、厚度、表面硬度和显微组织。结果表明:304奥氏体不锈钢在430℃渗氮不同时间后,渗氮层厚度和表面硬度都随着时间的延长而增加。渗氮时间为1 h时,渗氮层仅为单一的S相,随着渗氮时间的增加,渗氮8 h时开始有少量CrN生成,渗氮16 h时,渗氮层由大量CrN+S相两相混合。用电化学极化的方法评价耐蚀性能的结果表明:盐浴渗氮处理后耐Cl-点蚀性能得到了一定的改善,在430℃渗氮4 h,其耐蚀性能是最好的,优于没经过渗氮的试样,而在所有的渗氮试样中,渗氮8 h、16 h的试样耐点蚀性能较差。     

直流电场对35钢盐浴渗氮动力学影响

通过在盐浴渗氮中施加7.5 V的直流电场,在不同外热温度(545~575℃),不同保温时间(60~120 min)下对35钢进行盐浴渗氮,研究了直流电场对35钢盐浴渗氮动力学的影响。利用光学显微镜、X射线衍射仪对渗层的显微组织、厚度及物相进行了测试和分析。结果表明:直流电场对活性氮原子在基体内部的扩散有显著促进作用,提高渗氮速度,降低盐浴渗氮温度或缩短保温时间;直流电场盐浴渗氮的扩散系数都比常规盐浴渗氮提高约2倍,扩散激活能从常规盐浴渗氮的220 k J/mol降低到181 k J/mol,从而达到增加渗层厚度的显著效果。有无直流电场条件下盐浴渗氮,35钢渗层主要物相均为ε-Fe3N及少量γ'-Fe4N。     

盐浴氮碳共渗40Cr钢凸轮轴双联齿轮的变形控制

对40Cr钢凸轮轴双联齿轮加工工序、预先热处理工艺和检测结果进行分析,找到了造成齿轮内孔盐浴氮碳共渗畸变的主要因素.通过调整加工工序和预先热处理工艺,提高了齿轮心部强度,机加工应力得到了有效消除,从而使凸轮轴齿轮盐浴氮碳共渗畸变得到有效控制.     

Effects of rare earths in borax salt bath immersion vanadium carbide coating process on steel substrate

Various amounts of FeSiRe23, which were used as reducing agents, were added into typical borax salt bath used in thermal diffusion (TD) process to explore the effects of rare earths on borax salt bath vanadizing process and microstructure and properties of vanadium carbide coating. The effect results of rare earths on coating thicknesses at different process conditions showed that the right amount of rare earths could greatly accelerate vanadizing rate and that the accelerating effect worked mainly at the initial stage of TD process and decreased with increasing thickness of vanadium carbide layer. Moreover, the comparison of diffusion activation energies at various temperatures revealed that rare earths have optimal accelerating effect at 950 °C or so. The effect results of rare earths on coating microstructure and properties showed that the addition of rare earths into the borax salt bath can decrease grain size and improve fragility of vanadium carbide coating but decrease coating microhardness. And the electron probe microanalysis (EPMA) revealed that the addition of rare earths could decrease carbon concentration in the coating.     

Nitrogen redistribution in /γ-iron nitride compound layers upon annealing

An /γ^′-iron nitride compound layer grown on α-Fe at 823 K by gas nitriding shows a nitrogen redistribution upon annealing at 630 K: an until now not observed “backwards growth” of the γ^′-sublayer occurs, accompanied by a nitrogen enrichment of the remaining -sublayer. The observed thickness changes can be explained theoretically.     

50W540型无取向硅钢的时效行为

研究了50W540型无取向硅钢中MnS粒子在300～700℃时效处理0.75～24 h不同时间时效析出行为与铁损变化的关系.结果表明,α-Fe基体中过饱和的Mn、S原子以MnS粒子形式沿位错析出,Mn原子的扩散距离是析出行为的制约因素.700℃时效时过饱和度低、Mn扩散快而MnS粒子易粗化,铁损降低.400～600℃时效时过饱和度高、MnS粒子析出数量增多,铁损升高.400℃以下时效时过饱和度很高,但Mn扩散慢,使细小MnS粒子难以析出且铁损无明显变化.     

低温盐浴氮碳共渗304奥氏体不锈钢的结构与性能

304奥氏体不锈钢经过450℃盐浴氮碳共渗4 h处理后,表面较为平滑,表层主要由含氮的S相组成;硬度达到约780 HV,耐磨性也大为提高。其抗均匀腐蚀性较差,但其抗点蚀能力优于抛光不锈钢。     

Influence of time on the microstructure of AISI 321 austenitic stainless steel in salt bath nitriding

Influence of nitriding time on the microstructure and microhardness of AISI 321 austenite stainless steel was investigated, using a complex salt bath heat-treatment at low temperature, 430 °C. Experimental results revealed that after salt bath nitriding, a modified layer was formed on the surface of substrate with the thickness ranging from 2 μm to 30 μm with changing treating time. The nitrided layer depth thickened extensively with increasing nitriding time. The growth of the nitrided layer takes place mainly by nitrogen diffusion according to the expected parabolic rate law. Scanning electron microscopy and X-ray diffraction showed that in 321 stainless steel subjected to complex salt bathing nitrided at such temperature for less than 8 hours, the main phase of the nitrided layer was expanded austenite (S phase) by large. When the treatment time is prolonged up to 8 hours and more, S phase is formed and subsequently transforms partially into CrN, and then the secondary CrN phase precipitated. With treating time prolonged, more CrN precipitates formed along the grain boundaries in the outer part. In the inside part between the some CrN and the substrate, there is still a broad single S phase layer. All treatments can effectively improve the surface hardness.     

Corrosion of stainless steels and carbon steel by molten mixtures of commercial nitrate salts

The isothermal corrosion behavior of two stainless steels and a carbon (C) steel in mixtures of NaNO₃ and KNO₃ was evaluated to determine if the impurities found in commodity grades of alkali nitrates aggravate corrosivity as applicable to an advanced solar thermal energy system. Corrosion tests were conducted for approximately 7000 hours with Types 304 and 316 stainless steels at 570 °C and A36 C steel at 316 °C in seven mixtures of NaNO₃ and KNO₃ containing variations in impurity concentrations. Corrosion tests were also conducted in a ternary mixture of NaNO₃, KNO₃, and Ca(NO₃)₂. Corrosion rates were determined by descaled weight losses while oxidation products were examined by scanning electron microscopy (SEM), electron microprobe analysis (EPMA), and x-ray diffraction (XRD). The nitrate mixtures were periodically analyzed for changes in impurity concentrations and for soluble corrosion products. Results of these tests indicated that the short-term corrosion rates of the stainless steel specimens in many of the mixtures could be described in terms of parabolic kinetics. However, no single rate law could be assigned to the corrosion kinetics resulting from exposure in all of the mixtures. For engineering applications, corrosion rates over the entire exposure period are best described as linear with respect to time. In the binary nitrate mixtures, the annualized rates of metal loss were found to be between 6 and 15 μm/year for the stainless steel specimens at 570 °C depending on the particular mixture. Metal loss for the C steel specimens immersed in these same mixtures at 316 °C extrapolated to approximately 1–4 μm/year. SEM and XRD revealed that the complex, multiphase surface oxides formed on the stainless steel coupons were composed primarily of iron-chromium spinel, iron oxides, and sodium ferrite. Magnetite was the principal corrosion product formed on the carbon steel specimens. Overall, for the typical range of impurities in commercially available nitrate salts, corrosion rates for solar thermal energy applications remained acceptable for all of the materials examined.     

TWIP钢在不同温度变形的加工硬化行为

通过温控拉伸试验、光学显微镜、X射线衍射技术和透射电镜分析了在298、373、473、573 K温度下变形时,20Mn24Cr5Al2Ni2TWIP钢的力学性能和显微组织变化规律。结果表明,TWIP钢的强度随变形温度的升高而降低,伸长率在373 K变形时比298 K变形显著下降;在373～573 K变形时伸长率有上升趋势;温度升高,组织中形变孪晶的数量减少,孪晶交叉现象减弱。研究TWIP钢的加工硬化行为表明,TWIP钢在拉伸过程中的加工硬化指数n值随真应变的增加而增加,在低应变区温度升高n值增加。在298～373 K变形时,形变孪晶占主导作用,在473～573 K变形时,形变孪晶和动态应变时效共同作用。     

Nitriding of Fe-Cr-Ni films by low energy ion implantation

Nitrogen low energy implantation was carried out simultaneously into thin austenitic stainless steel films, deposited by ion-beam sputtering, and bulk samples with the aim to investigate the influence of the grain size and microstructure on diffusion and phase formation. Nitrogen uptake, diffusion and phase formation were investigated using SIMS, XRD and TEM. The diffusion itself is very similar in bulk material and thin films, indicating that the grain size differing by close to a factor of 1000 is not the dominating factor. In contrast, the transition towards CrN precipitates within a martensitic host was only found for the thin films implanted at 360 °C.     

42CrMo钢在不同温度下单向拉伸的流变特征

对42CrMo调质钢在-150～700℃区间进行拉伸试验,发现在一定应变条件下出现应力突然增大的"应力台阶"现象,"应力台阶"的高度与试验温度存在一定关系。随着温度提高,流变应力及强度均降低,但是在0～250℃区间出现应力平台。流变应力随温度的变化与位错同固溶原子之间的交互作用有关。在低温条件下,钢的断裂方式以解理为主,随着温度提高逐渐过渡到韧窝断裂。     

IF钢铁素体区轧制的变形抗力模型

利用Gleeble热模拟试验机对IF钢铁素体区轧制的变形抗力进行了试验研究。通过实测铁素体区不同变形温度、应变速率、变形程度和变形抗力的关系,建立了变形抗力的数学模型。通过对模型进行回归分析,证明该模型具有良好的曲线拟合特性,可为IF钢铁素体区轧制力能参数计算提供准确的数学模型。     

Surface microstructure of 316L austenitic stainless steel by the salt bath nitrocarburizing and post-oxidation process known as QPQ

Systematic materials characterization of the Quench-Polish-Quench complex salt bath heat-treatment process (QPQ) surface modified 316L steel was investigated. The results reveal that the nitro-carburized sample surfaces consist of Cr₂N/γ´-Fe₄N and CrN/γ-Fe, while the post-oxidized sample surfaces are comprised of CrO₃, (Fe₃O₄)/ε-Fe₂(N,C), γ′-Fe₄N and S(γ_N)/CrN/α-Fe. Nuclei ε-Fe₂N_1 − x accumulates at the interface between oxide layer and nitride compound layer by the help of post-oxidation. The diffraction peak lines of S-phase (γ_N) move gradually towards higher diffraction angles as increasing depth of 15 μm to 35 μm. An increased content of oxygen is recorded in the post-oxidized surface layer down to the depth of approximately 15 μm, a small peak concentration of carbon occurs at the front of the nitrided layer. Micro-hardness of the post-oxidized samples reaches about 1300 HV_0.1 near the surface region and then reduces sharply across the case/substrate interface.