渗氮及表面增碳对3Cr2W8V钢冷热疲劳行为的影响

用板状缺口试样研究了在自约束条件下，表面增碳和渗氮对３Ｃｒ２Ｗ８Ｖ钢冷—热疲劳裂纹萌生，扩展及裂纹形态特征的影响，结果表明，表面增碳及渗氮均使钢的热疲劳裂纹萌生及扩展抗力降低，渗氮尤为显著，经渗氮处理的试样在冷—热疲劳试验中除缺口尖端出现主裂纹外，试样表面较早出现大量裂纹，最后形成龟裂；表面增碳的试样在试验中也出现少量表面裂纹，而表面化学成分无变化的试样则未出现表面裂纹。     

Development of in-situ composite surface on mild steel by laser surface alloying with silicon and its remelting

In the present study, an attempt has been made to develop in-situ iron silicide dispersed surface on mild steel substrate by laser surface alloying with silicon using a high-power continuous wave CO₂ laser. The effect of laser surface remelting of the alloyed surface using argon and nitrogen (with and without graphite coating) as shrouding environment has also been studied. The microstructure of laser surface alloyed mild steel with silicon consists of uniformly dispersed iron silicide in grain refined α-iron matrix with an improved microhardness to 575 VHN as compared to 150 VHN of as-received mild steel substrate. Surface remelting in Ar atmosphere coarsened the microstructure and reduced the area fraction of silicide and hence, reduction in the microhardness to 450 VHN. Surface remelting in nitrogen increased the microhardness to 740 VHN due to the formation of iron nitrides in addition to the presence of silicides. Graphite coating prior to remelting improved the microhardness to 800 VHN due to the presence of martensites along with nitrides and silicides. A maximum enhancement in wear resistance was achieved when remelting was done in nitrogen environment with carbon deposition. The mechanism of wear was found to be predominantly abrasive in nature as compared to adhesive and oxidative in as-received mild steel.     

Effect of strengthening friction treatment on the chemical composition,structure, and tribological properties of a high-carbon steel

The structure and chemical composition of nanocrystalline layers formed on the surface of a steel U8 with 0.83 wt % C (quenched, as well as quenched and tempered at 200°C) under the conditions of frictional loading by a hard-alloy indenter in different media (gaseous and liquid nitrogen, air) have been investigated by the methods of transmission electron microscopy, X-ray diffraction analysis, nuclear reactions, Rutherford back scattering, and wave- and energy-dispersive microanalyses. Maximum levels of defectiveness (high density of dislocations and point defects) and microhardness of the nanocrystalline structure have been attained upon friction treatment of the low-temperature tempered steel in a liquid-nitrogen medium because of deformation localization in a thin surface layer, intensification of deformation-induced dissolution of the ɛ carbide phase, and saturation of the layer with nitrogen and oxygen atoms, the latter dissolved in the liquid nitrogen as an impurity. A comparative analysis of the in-depth distribution of microhardness in frictionally strengthened surface layers has been performed for the steel with initial structures of tetragonal (untempered) and low-tempered (tempered at 200°C) martensite. A markedly larger depth of strain hardening has been attained upon friction treatment in the quenched untempered steel due to effective development of deformation-induced dynamic aging of high-carbon martensite even at small deformations. It has been established that the strengthening upon deformation of the surface by a sliding indenter exerts a positive influence on the tribological properties (wear rate and friction coefficient) of the steel under the conditions of frictional heating of different intensity.     

Structure,mechanical, and tribotechnical properties of an austenitic nitrogen steel after frictional treatment

The features of the structure and the mechanical and tribotechnical properties of an austenitic nitrogen steel (Cr16.5, Mn18.8, C0.07, N0.53, Si0.52 wt %, and Fe for balance) after frictional treatment have been studied. It has been shown that, along with twinning, the nitrogen austenite upon frictional treatment undergoes a γ → stacking fault → ε transformation. The strengthening of the steel by the frictional treatment manifests in a delay of the onset of the plastic flow. In the structure of the surface layer with a thickness of 5 μm, a high concentration of stacking faults has been detected. The mechanical properties depend on the orientation of the acting stresses relative to the direction of the frictional treatment. Upon the sliding friction of a ball made of hard alloy (94%WC + 6%Co) on the strengthened surface, an anomalously low coefficient of friction of 0.13 is observed. The coefficient of friction in the presence of abrasive particles in the form of wear debris increases to 0.50; however, the wear rate is almost two times lower compared to the same characteristic for a nonstrengthened surface of the nitrogen steel tested under the same conditions.     

Nitriding of steel in potassium nitrate salt bath

A potassium nitrate salt bath has been used for nitriding of interstitial-free steel. The nitriding behavior can be reasonably well described by nitrogen diffusion in iron. Most nitrogen is dissolved interstitially. During nitriding, a slight oxidation of the steel surface also takes place. The nitrided specimen achieves a pronounced solid–solution strengthening.     

Kinetics of nitrogen absorption in molten aisi 316 stainless steel during immersion nitrogen blowing

Nitrogen absorption in molten metal for stainless steel AISI316 has been investigated by immersion nitrogen blowing through an immersed alumina nozzle with an internal diameter of 3 mm. Based on these experimental data, some kinetic parameters of nitrogen absorption, such as reaction order, rate constant and apparent activation energy of nitrogen absorption reaction, have been obtained. Effect of stirring by immersion nitrogen blowing through an immersed alumina nozzle on nitrogen absorption reaction has been observed. Results show the following: (1) Nitrogen absorption reaction is the −1.5th order reaction. The rate constant k _N is of the order of 10⁻⁵ wt%^2.5·min⁻¹. Nitrogen absorption reaction for AISI 316 has negative apparent activation energy of −92.40 kJ·mol⁻¹. This indicates that the nitrogen absorption reaction has a complex and multistep reaction mechanism. (2) The rate of nitrogen absorption reaction in molten stainless steel is mixture control by the adsorption of monatomic nitrogen on the surface of molten stainless steel and mass transfer in molten metal. (3) A rate equation of nitrogen absorption reaction has been derived based on a mixed control mechanism by both the -1st order nitrogen absorption reaction and mass transfer in molten metal.     

Formation and microstructural characterisation of S-phase layers in Ni-free austenitic stainless steels by low-temperature plasma surface alloying

The feasibility of generating S-phase surface layers in nickel-free austenitic stainless steels by plasma surface alloying with nitrogen (at 430 °C), carbon (at 430 °C and 500 °C) and both carbon and nitrogen (at 430 °C) has been investigated. The structure, microstructure and composition of the plasma-alloyed surfaces were characterised by X-ray Diffraction (XRD), microscopy, Glow Discharge Optical Emission Spectroscopy (GDOES) and Transmission Electron Microscopy (TEM). The experimental results have demonstrated for the first time that the S-phase can be produced in the surface of nickel-free austenitic stainless steel by low-temperature plasma surface alloying. TEM analysis has revealed that when alloyed with carbon no precipitates can be found within the carbon-rich S-phase layer; however, when alloyed with nitrogen or both carbon and nitrogen some nitride precipitates (Mn₃N₂ and Cr₂N) were found within the nitrogen-rich S-phase layer. Based on experimental results, the response of Ni-free austenitic stainless steel to plasma surface alloying has been compared to the Ni-containing counterpart, and the role of nickel in the formation of S-phase in austenitic stainless steels has been discussed.     

等离子体冶炼的N─Cr─Mn钢的耐腐蚀性能

用等离子氮弧冶炼不同氮含量的铬锰不锈钢,其耐腐蚀性能用动电位扫描、电化学阻抗等方法测试,并与1Cr18Ni9不锈钢作比较,结合显微组织分析耐蚀原因。实验结果表明含氮钢的耐蚀性普遍好于1Cr18Ni9不锈钢。随钢中含氮量增加,奥氏体组织扩大,耐腐蚀性能提高。当含氮量达0.74%时形成完全的奥氏体相。经阳极极化后,含氮钢表面的腐蚀层中奥氏体相比原基体的扩大,膜致密,耐腐蚀性能远远高于原基体。     

Influence of processing conditions on structural characteristics of hybrid plasma surface alloyed austenitic stainless steel

A hybrid plasma surface alloying process has recently been developed for austenitic stainless steels to improve their surface hardness, wear resistance and corrosion resistance. The process is carried out in nitrogen and methane gas mixtures at temperatures below 450°C and facilitates the simultaneous incorporation of nitrogen and carbon into the surfaces of austenitic stainless steels, forming a dual layer structure with an extremely hard nitrogen-enriched layer on top of a hard carbon-enriched layer. The present paper discusses the influence of three most important processing parameters, i.e., gas composition, temperature and time, on the structural characteristics of the alloyed zones produced on AISI 321 stainless steel, in terms of layer morphology, growth kinetics and chromium compound precipitation. It was found that the development of the alloyed layers is diffusion-controlled, and under proper processing conditions, a precipitation-free dual-layer structure can be produced, with nitrogen and carbon dissolved in the relevant layer forming N-expanded austenite and C-expanded austenite respectively. Based on the experimental results, a threshold temperature-time curve has been established for the investigated austenitic stainless steel.     

精密铸钢件表面脱碳的防止措施

在浇注前和浇注后分别向精密铸造型壳内通入氮气或丙烷气,采用不填砂浇注方法获得铸件。观察铸件表层组织,并进行硬度测试,分析采用各种方法减轻脱碳现象的原因。结果表明,通入两种气体均可以起到减轻或防止精铸件表面脱碳的现象,其中丙烷气防脱碳的效果较好,通入的气体占据了整个型腔,赶走了空气,隔绝外界空气的侵入,避免脱碳反应的发生,从而有效地防止了精铸钢件表面脱碳现象的发生。     

Corrosion behaviour of Lotus-type porous high nitrogen nickel-free stainless steels

Corrosion behaviour of three austenitic Lotus-type porous high nitrogen Ni-free stainless steels exposed to an acidic chloride solution has been investigated by electrochemical tests and weight loss measurements. Polarization resistance indicates that the corrosion rate of Lotus-type porous high nitrogen Ni-free stainless steels is an order of magnitude lower than that of Lotus-type porous 316L stainless steel in acidic environment. The localised corrosion resistance of the investigated high nitrogen Ni-free stainless steels, measured as pitting potential, E_b, also resulted to be higher than that of type 316L stainless steel. The influences of porous structure, surface finish and nitrogen addition on the corrosion behaviour were discussed.     

On the mechanism of ion nitriding of an austenitic stainless steel

The mechanism of nitrogen transport has been investigated in an austenitic stainless steel (AISI 304) under high flux and low energy nitrogen ion beam irradiation at moderate temperatures in the range 270–550°C. The profiles of the distribution of nitrogen have been analyzed with nuclear reaction analysis (NRA) and glow discharge optical spectroscopy (GDOS), and the surface roughness with scanning AFM. The modeling is based on the study of the stochastic mixing of atoms ‘ballistically’ displaced by incident ions. The development of surface roughness and the formation of an altered layer highly enriched by nitrogen are analyzed, and it is concluded that the transport of nitrogen into the bulk results from a flux of matrix atoms driven by mobile vacancies at temperatures above 350°C. This behavior is consistent with an altered layer ‘growth’ that is controlled by the ion-beam-induced displacements of surface atoms.     

The influence of surface microcracks and temperature gradients on the rf plasma nitriding rate

Rf plasma nitriding processes have been used recently for surface treatment. 304 Austenitic stainless steel has been nitrided using an inductively coupled rf plasma. High rates of nitriding and relatively high microhardness have been achieved. The effect of plasma power and plasma processing time on the rate of nitriding and the microstructure was investigated. A high nitriding rate of 3 μm²/s and a microhardness value of 1800 HV0.1 were achieved under optimum plasma conditions. The interpretation is based on a new approach that the microcracks in the surface, at a certain ambient temperature and plasma, can create traps for the active nitrogen species. The nitrogen concentration and temperature gradient can also draw some of the nitrogen species towards the bulk side. The microhardness can be explained in terms of the high concentration of nitrided phases created under the surface of the treated samples.     

30CrMnSiA钢渗扩氮高温淬火及渗扩氮亚温淬火研究

研究了预渗氮、渗扩氮高温淬火及平衡态原始组织渗扩氮亚温淬火、低温回火对结构钢３０ＣｒＭｎＳｉＡ的显微组织和显微硬度分布曲线的影响规律，观察了高氮区和低氮区的含氮马氏体形态和亚结构。     

等离子体渗氮气压对合金钢渗氮层微观结构和性能的影响

目的 选择M50NiL钢（高合金钢）和AISI 4140钢（低合金钢）2种合金钢,研究渗氮气压对合金钢等离子体渗氮层组织结构、渗层厚度、硬度、韧性和摩擦磨损性能的影响规律。方法 根据离子渗氮GB/T30883—2017,在0～500 Pa渗氮气压范围内选择170、250、350 Pa 3个渗氮气压进行等离子体渗氮,研究渗层微观结构和性能。结果 对于M50NiL和AISI 4140两种合金钢,350 Pa时渗层厚度均最大,170 Pa次之,250 Pa厚度最小。M50NiL钢在350 Pa渗氮和AISI 4140钢在170 Pa渗氮时,表面层具有最优的强韧性。摩擦磨损性能显示,170 Pa和350 Pa气压渗氮的摩擦磨损性能明显优于250 Pa气压渗氮,其中磨损率规律与渗氮层的韧性值测试结果吻合。结论 气压影响了氮离子的能量和分布,从而影响了渗层厚度,钢中的合金元素含量和气压共同影响表面强韧化效果,并且表面强韧化效果直接影响渗氮层的摩擦磨损性能。     

Improvement of the Corrosion Resistance of High Alloyed Austenitic Cr-Ni-Mo Stainless Steels by Solution Nitriding

Characteristic features of austenitic steel grades combine a good corrosion resistance with a low hardness, wear resistance and scratch resistance. An interesting possibility for improving the wear behaviour of these steels without loss of their corrosion resistance lies in enriching the near surface region with nitrogen. The process of a solution nitriding allows the rise of the solution of nitrogen in the solid phase. On this state nitrogen increases the corrosion resistance and the tribilogical load-bearing capacity. The aim of the study was, to investigate the improvement of the pitting corrosion behaviour by solution nitriding. A special topic was to observe the effect of nitrogen by different molybdenum content. So austenitic stainless steels (18% Cr, 12% Ni, Mo gradation between 0.06 to 3.6%) had been solution nitrided. The samples could be prepared with various surface content of nitrogen from 0.04 to 0.45% with a step-by-step grinding. The susceptibility against pitting corrosion of these samples had been tested by determination of the stable pitting potential in 0.5M and 1M NaCl at 25℃. For the investigated steel composition and the used corrosion system there is no influence of molybdenum on the effectiveness of nitrogen. The influence of nitrogen to all of the determined parameters can be described well by PRE = Cr 3.3 * Mo 25 *N. XPS analysis of the sample surfaces support the results of the pitting corrosion tests.Additionally surface investigations with an acid elektolyte (0.1M HCl 0.4M NaCl) were performed. In this case the passivation effective nitrogen content increases markedly with rising molybdenum concentration of the steel.Obviously an interaction of Mo and N is connected with a strongly acid electrolyte.     

烧结材料孔隙形态与分布对离子氮碳共渗层组织结构的影响

研究了烧结材料孔隙形态与分布对离子氮碳共渗层组织结构的影响。试验表明，随试样孔隙数量的增加，γ＇相含量和扩散层深度增加。由于试样的正压制面与侧压制面的孔隙形态及分布不同，导致化合物层深度和相组成不同。还观察讨论了氮原子在氮碳共渗层的分布状态。     

IGSCC crack growth in simulated BWR environment – Effect of nitrogen content in non-sensitised and warm rolled austenitic stainless steel

Effect of nitrogen level in strain hardened stainless steel (SS) on crack growth rate (CGR) in simulated boiling water reactor conditions has been the focus of this study. Type 304 LN stainless steel has been used in a warm rolled condition containing two different levels of nitrogen. Clear intergranular (IG) fracture was observed in both the stainless steels. The CGR increased 3 times in the stainless steel with higher level of nitrogen at all levels of dissolved oxygen and this was related to the increase in yield strength due to rolling and dynamic strain aging (DSA).     

Austenite modification of AISI 316L SS by pulsed nitrogen ion beams generated in dense plasma focus discharges

We present the results of a surface modification of AISI 316L stainless steel by surface irradiation with high energy, pulsed nitrogen ion beams generated with 0.8 kJ dense plasma focus. The surface characterization was done using GAXRD, Auger electrons spectroscopy, TEM and optical microscopy. After the irradiation, we found a modification of a 1 μm thick surface layer, on which a gradual lattice expansion of the austenite with the number of irradiation pulses, i.e. with the total nitrogen ion fluence, was observed.In addition, ~ 40 nm close to the surface layer, a disordered lattice structure had been observed through TEM analysis. Those results can be explained in terms of the extreme thermal effect induced on the surface through the fast high energy release during the pulsed ion interaction with the steel surface, followed by an also rapid cooling down process which limits the nitrogen diffusion to the bulk.     

氮气孔的消除

通过对氮气孔形貌的分析,找出了氮气孔形成的主要原因。通过如下措施,彻底解决了氮气孔缺陷问题：（1）控制废钢加入量在45%以下;（2）少用或不用螺纹钢、导轨钢等w（N）量高的废钢;（3）用加入适量钛铁的方法降低铁液中的w（N）量;（4）在用感应电炉熔化时,保证w（N）量不低于60ppm。