Degradation of steel in gaseous hydrogen with inhibiting admixtures

We have investigated the possibility to protect metal constructional elements of power-generating equipment against hydrogen embrittlement by introducing O₂ and CO + H₂ O admixtures into the working volume. We have shown their positive influence on the mechanical properties of 03Kh12N10MT steel and KhN60K16MBYu alloy under short-term and long-term static tension within the temperature range 293–1073 K in gaseous hydrogen with a pressure of 35 MPa. However, we have not detected an inhibiting effect of the admixtures mentioned above in the case of preliminary high-temperature hydrogenation of unloaded smooth and notched specimens made of 05KhN23MTR steel and KhN60K16MBYu alloy. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 71–75, May–June, 2007.     

Improving the wear resistance of M41 steel by nitrogen alloying and ESR

AISI M41 high-speed steel (HSS) is considered as a super-hard tool steel due to high hardness level (65–70 HRC). Nitrogen alloying of AISI M41 HSS produces marked solid solution hardening and precipitation strengthening in addition to an improvement in pitting resistance. The mechanical properties in general, and wear resistance in particular, are strongly affected by the steel cleanliness and the status of non-metallic inclusions in steel. For this reason tool steel should be subjected to a secondary refining process. In this work, the wear characteristics of AISI M41 HSS were investigated. The effect of nitrogen alloying and electroslag refining (ESR) of this steel grade were considered. Both conventional and nitrogen alloyed grades were melted in open air induction furnaces and then remelted under three different compositions of calcium fluoride-based flux in an ESR machine. The wear behaviour of the resulting steels, for both conventional and nitrogen-alloyed grades before and after ESR, was monitored. The addition of nitrogen improves markedly the wear resistance of AISI M41 HSS. This improvement depends on the total nitrogen content and is independent on the form of the nitrogen constituent. The ESR process improves markedly the wear resistance of both conventional and nitrogen-alloyed grades.     

Microhardness profile prediction of a graded steel by strain gradient plasticity theory

In the present study, the Vickers microhardness profile of functionally graded steel austenitic steel produced by electroslag remelting process has been investigated. To produce functionally graded steels, two different slices from plain carbon steel and austenitic stainless steels were spot welded and used as electroslag remelting electrode. Functionally graded steel containing graded layers of austenite may be fabricated via diffusion of alloying elements during remelting stage. Vickers microhardness profile of the specimen has been obtained experimentally and modeled with mechanism-based strain gradient plasticity theory. In this regard, the density of the statistically stored dislocations and that of geometrically necessary dislocations was related to the Vickers microhardness profile of each layer. The experimental results are in good agreement with those obtained from the theory.     

强化研磨微纳加工参数对轴承套圈滚道表面硬度的影响

目的 探明不同加工参数对加工表面平均硬度的影响规律。方法 用强化研磨微纳加工技术对6012深沟球轴承内圈滚道表面进行强化处理,通过设置不同的喷射压力、加工时间、喷射角度及钢珠配比获得加工试样。采用洛氏硬度计分别检测加工前后套圈滚道表面硬度,并分析其随各参数变化的规律。结果 加工时间为5 min,喷射角度为45°,喷射压力为0.4～0.6 MPa时,表面硬度随喷射压力的增大而增大,0.6 MPa后维持在HRC61.60附近;喷射压力为0.6 MPa,喷射角度为45°,加工时间为1～5 min时,表面硬度及其增量随时间增加而增大,其后在HRC61.50附近徘徊;喷射压力为0.6MPa,加工时间为5min,喷射角度为35°～55°时,试样表面硬度及其增量先减小后增大,喷射角度达50°后,表面硬度达最大值HRC63.45;钢珠配比则对试样表面硬度影响不大,加工所得试样在HRC61.67～HRC61.80之间。结论 试样表面硬度及其增量随喷射压力、加工时间及喷射角度的增加而增加,且受加工时间影响最大。当加工时间为5 min,喷射压力为0.6 MPa,喷射角度为50°时,可获得较高的平均表面硬度。     

Effects of alloying elements and cooling rates on the high-strength pearlite steels

The effects of the alloying elements of Cr, Mn and the cooling rates after hot deformation on the microstructures and mechanical properties of pearlite steels were studied. Results show that increasing Cr and decreasing Mn significantly increase the eutectoid transformation temperature of steel. The grain sizes of prior austenite of the steels after hot deformation are ～12?µm. However, the high-Cr–low-Mn steel exhibits a finer interlamellar spacing and some better mechanical properties than that of the high-Mn–low-Cr steel. A full pearlite microstructure with an interlamellar spacing of 97?nm was obtained on the former steel, which exhibits a hardness of HRC49, a tensile strength of 1700?MPa and an elongation of 19%.     

Development of wear resistant composite surface on mild steel by laser surface alloying with silicon and reactive melting

The present study concerns laser surface alloying with silicon of mild steel substrate using a high-power continuous wave CO₂ laser with an objective to improve wear resistance. The effect of surface remelting using nitrogen as shrouding environment (with and without graphite coating) on microhardness and wear resistance has also been evaluated. Laser surface alloying leads to formation of a defect free microstructure consisting of iron silicides in laser surface alloyed mild steel with silicon and a combination of silicides and nitrides when remelted in nitrogen. Carbon deposition prior to remelting leads to presence of a few martensite in the microstructure. A significant improvement in microhardness is achieved by laser surface alloying and remelting to a maximum of 800 VHN when silicon alloyed surface is melted using nitrogen shroud with carbon coating. A detailed wear study (against diamond) showed that a significant improvement in wear resistance is obtained with a maximum improvement when remelted in nitrogen atmosphere followed by carbon coating.     

Fretting fatigue behaviour of Ni-free high-nitrogen stainless steel in a simulated body fluid

Abstract

Fretting fatigue behaviour of Ni-free high-nitrogen steel (HNS) with a yield strength of about 800 MPa, which was prepared by nitrogen gas pressurized electroslag remelting, was studied in air and in phosphate-buffered saline (PBS(-)). For comparison, fretting fatigue behaviour of cold-rolled SUS316L steel (SUS316L(CR)) with similar yield strength was examined. The plain fatigue limit of HNS was slightly lower than that of SUS316L(CR) although the former had a higher tensile strength than the latter. The fretting fatigue limit of HNS was higher than that of SUS316L(CR) both in air and in PBS(-). A decrease in fatigue limit of HNS by fretting was significantly smaller than that of SUS316L(CR) in both environments, indicating that HNS has better fretting fatigue resistance than SUS316L(CR). The decrease in fatigue limit by fretting is discussed taking into account the effect of friction stress due to fretting and the additional influences of wear, tribocorrosion and plastic deformation in the fretted area.     

Effect of Hydrogen on the Mechanical Properties of Welded Joints of 03Kh12N10MT Steel and KhN55MBYu Alloy

We have studied the hydrogen degradation of welded joints of 03Kh12N10MT steel and KhN55MBYu alloy, used in the aerospace industry. We have established that the highest level of strength, plasticity, and low-cycle durability in gaseous hydrogen under a pressure of 35 MPa is provided by argon-arc welding for joining homogeneous materials and by electron-beam welding for dissimilar materials.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 55–61, November–December, 2004.     

Microstructure and property evolutions of a novel Super304H steel during high temperature creeping

High-temperature creep tests of a novel Super304H steel under 650 °C/195 MPa were conducted and the evolutions of microstructure and property with creep time of the material were investigated by using optical microscope, scanning electron microscope, micro Vickers hardness tester and electrochemical workstation. The results show that M₂₃C₆ carbides precipitated along grain boundaries of austenite matrix in a chain distribution and then got coarsened with the increase of creep time. Creep cavities started to form near the surface when the steel was crept for 2500 h. Afterward creep cavities increased, developed, interconnected and finally formed micro cracks along grain boundaries till fracture at the time of 4578 h. The hardness of the steel increased dramatically at the early stage of creeping and reached a high level at 500 h, and then kept a stable state at the succedent stage till fracture. Intergranular corrosion susceptibility of the steel increased first and then declined gradually, indicating the occurrence of sensitization – desensitization process of the steel during creeping.     

Fatigue properties of two case hardening steels after carburization

Fatigue properties of two case hardening steels after carburization have been investigated by means of rotating bending fatigue tests and rolling contact fatigue tests. Results show that the steel with higher Al and N contents has higher rotating bending fatigue limit and rolling contact fatigue limit, increasing from 865 to 950 MPa and from 3575 to 3725 MPa, respectively. It is also shown that the steel with higher Al and N contents has finer prior austenite grain sizes and higher hardness in the carburized case. Scanning electron micrographic observations on the fractured surface of specimens for rotating bending fatigue tests show that fatigue crack usually initiated from oxide inclusions and propagated along prior austenite grain boundaries, indicating that the finer grain size and higher hardness in the carburized case of the steel with higher Al and N contents can contribute to its higher fatigue properties.     

分级固溶处理对8Cr4Mo4V钢的微观组织和硬度的影响

对8Cr4Mo4V航空轴承钢进行分级固溶处理,即在1000~1060℃的初级固溶处理和在1080~1100℃的二级固溶处理,并观察和测试其组织和硬度,研究了分级固溶温度的影响。结果表明,随着初级固溶温度的提高(二级固溶处理为1080℃×10 min),钢中未溶碳化物的体积分数从4.37%逐渐降低到3.43%,但是晶粒没有明显长大。随着二级固溶温度的提高(初级固溶处理为1060℃×30 min),未溶碳化物的体积分数从3.51%逐渐降低到2.84%,平均晶粒尺寸显著增大。当初级固溶温度较低或二级固溶温度较高时,8Cr4Mo4V钢的回火硬度较高。为了使8Cr4Mo4V钢具有高硬度同时避免晶粒粗化,初级固溶温度宜为1020~1050℃,二级固溶温度宜为1080~1090℃。对这种钢进行1020℃×20 min+1090℃×10 min固溶处理后,其平均晶粒尺寸为12.1 μm,回火硬度为63.8 HRC,冲击吸收功为15.28 J,室温抗拉强度为2664.3 MPa。     

Analysis of Magnetism in High Nitrogen Austenitic Stainless Steel and Its Elimination by High Temperature Gas Nitriding

Stable austenitic structure in medical stainless steels is basically required for surgical implantation. A weak magnetism was found in a high nitrogen nickel-free austenitic stainless steel for cardiovascular stent application. This magnetic behavior in high nitrogen stainless steel was investigated by optical microscopy, X-ray diffraction (XRD), electron probe microanalysis (EPMA) and superconducting quantum interference device (SQUID). The results showed that the magnetism came from the composition segregation of ferrite formation elements such as Cr and Mo in the steel and some δ-ferrites were locally formed during the pressurized electroslag remelting process. The magnetism of high nitrogen stainless steel could be eliminated by a proper high temperature gas nitriding (HTGN).     

A method for the prevention of premature failures of power plants using hydrogen as the working medium

Qe analyze failures of power-generating units working on liquid hydrogen caused by the most dangerous type of embrittlement—reversible hydrogen embrittlement of the metal. We consider the existing methods for the protection of various structural elements against hydrogen embrittlement and discuss their advantages and shortcomings. A method for the protection of difficultly accessible internal hollows of power-generating units against hydrogen embrittlement by introducing active gaseous admixtures in the working volume is described in detail. The mechanism of action of these admixtures is analyzed refractory alloy as examples, we demonstrate the positive effect of admixtures to gaseous hydrogen under a pressure of 35 MPa. The proposed method enables one, in some cases, to use hydrogen (as a new ecologically pure fuel) without serious changes in the structure of the unit and in the technological process. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 53–56, July–August, 1998     

Nichtmagnetisierbarer warmbeständiger nichtrostender Stahl für Wälzlager

Nonmagnetic heating‐resistant stainless steel for roller bearings A low cost austenitic chromium manganese steel with about 1 mass% of carbon and nitrogen was molten under normal pressure which reveals an amazing combination of properties. Starting from a yield strength of about 600 MPa it is cold work hardened to 60 HRC. This high hardness is brought about for the first time without a martensitic microstructure which is usual for roller bearings. In addition this steel is stainless, non‐magnetic and heating resistant up to about 500 °C, i.e. a material to serve under complex loading. Manufacturing by ingot metallurgy, ESR, hot working, solution annealing and machining was carried out on an industrial scale. The investigation of the structure was carried out on several scales, beginning with the electronic structure, the TEM structure, the light optical microscopy up to macro‐etchings. In this manner an extensive understanding of the outstanding combination of properties of the steel named CARNIT was derived.     

Estimating the viability of a tension bolt in a gas turbine

An estimate has been made of the crack growth period in an M200 x 4 mm coupling bolt made of 26KhN3M2FA steel in a gas turbine operating at 293–523 K under fewcycle loading arising from thermal expansion of the disks together with the dimensions of nonpropagating fatigue cracks under many-cycle loading due to vibration. The critical ring crack depth in a 26KhN3M2FA steel bolt is 26 mm at a stress of 320 MPa. The crack develops to a critical size from a depth of 10 mm during 30,000 start-stop cycles.Translated from Problemy Prochnosti, No. 9, pp. 53–57, September, 1992.     

铸锭BFe30-1-1锻造性能的研究

研究了氧含量、夹杂物、宏观组织、拉制速度和电渣重熔等因素对BFe10 1 1合金铸锭锻造性能的影响,结果表明:氧含量、夹杂物对锻造性能影响不大,铸锭中的粗大晶粒及存在的缺陷将会降低锻造性能,而斜向生长的柱状晶区有利于锻造的质量控制,慢速停拉式铸造和合理的电渣重熔工艺对发挥合金的可锻性有利。     

Laser surface alloying of case hardening steel with tungsten carbide and carbon

Abstract

The laser surface alloying process was used to introduce two different alloying materials, tungsten carbide (WC/Co) and carbon, into the molten surface of a case hardening steel (16MnCrS5), to improve its hardness and wear resistance. The chemical composition and the resulting microstructure in the alloyed layers were of particular interest in this investigation, because the strengthening mechanism was strongly dependent upon the type and amount of the alloy material. For laser alloying with carbon the increase in hardness and wear resistance was based on the martensitic transformation in the composition range concerned. For alloying with tungsten carbide it was necessary to consider two different strengthening mechanisms, namely, martensitic transformation and precipitation of carbides. In both cases the grain refinement in the laser affected zone had an additional effect. Resistance to dry abrasive sliding wear was measured using a conventional pin-on-disc wear testing machine. For both alloy materials the wear rate was substantially lower than that of a substrate that had been laser remelted without alloying additions.

MST/1556     

Evaluation of the Electroslag Remelting Process in Medical Grade of 316LC Stainless Steel

This study is focused on the effects of electroslag remelting by prefused slag(CaO,Al2O3,and CaF2)on macrostructure and reduction of inclusions in the medical grade of 316LC(316LVM)stainless steel.Analysis of the obtained results indicated that for production of a uniform ingot structure during electroslag remelting, shape and depth of the molten pool should be carefully controlled.High melting rates led to deeper pool depth and interior radial solidification characteristics,while decrease in the melting ra...     

A study of self‐hardening bainitic cast steel

Bainitic cast steel is a kind of wear resistant material which has high strength and toughness, and can usually be obtained by isothermal quenching or molybdenum alloying. However, isothermal quenching has lower production efficiency and molybdenum alloying has higher production cost. In this paper, according to the characteristics that manganese and boron elements delayed the pearlitic transformation, the authors developed a new type of self‐hardening bainitic cast steel in which manganese and boron were main alloy elements and a small amount of titanium, nitrogen, calcium, barium and yttrium elements were also added in the steel that could refine and purify the solidification structure of steel. On this basis, the author studied the effect of tempering treatment on microstructures, mechanical properties and wear resistance of bainitic cast steel. The results showed that impact toughness of bainitic cast steel increased ceaselessly with the increase of tempering temperature, and there was tempering brittleness while tempering from 450°C–500°C. Moreover, the hardness of bainitic cast steel decreased with the increase of tempering temperature, and hardness decreased slowly and maintained at 55HRC or above when tempering temperature was lower than 300°C. Under the condition of two‐body pin‐on‐disc wear, the wear resistance of bainitic cast steel decreased with the increase of tempering temperature, but bainitic cast steel tempering at 300°C had excellent wear resistance in the condition of impact wear. In the practical use, the bucket teeth of excavator and the hammer of crusher making from self‐hardening bainitic cast steel were safe and reliable, and their service life were increased by 120–150% than Hadfield manganese steel.     

低碳中合金钢破碎机锤头的研制

研究了一种新型的破碎机锤头,对其采用合金化处理并经过热处理,实验表明:该低碳中合金耐磨钢的硬度大于51HRC,冲击韧性大于141J/cm2。经热处理后的显微组织为回火马氏体+残余奥氏体+弥散的碳化物,比高锰钢破碎机锤头性能有很大提升。