The microstructure and phase relationships in rapidly solidified type 304 stainless steel powders

The microstructure and relative amounts of fcc and bcc phases have been studied for rapidly solidified Type 304 stainless steel powders produced by vacuum gas atomization (VGA) and centrifugal atomization (CA). The VGA powder solidifies with a cellular microstructure while the CA powder has a dendritic microstructure. The volume fraction of fcc phase in the CA powder is found to increase from 40 Pct to 97 Pct with increasing particle size from 30 to 125 μm. In the VGA powder, the volume fraction of fcc phase is found to decrease from about 90 Pct to 77 Pct over the same range of particle sizes. The origins of the fcc and bcc phases in each powder are considered. It is concluded that bcc is present as both a primary crystallization phase in the smaller CA particles (<75 μm) and as compositionally stabilized eutectic ferrite at the cell walls of particles of both CA and VGA powders in which fcc was the primary crystallization phase.     

粉末形状与松装密度对不锈钢烧结多孔材料制备工艺及其性能的影响

通过选用气雾化及水雾化两种工艺方法制备的不锈钢粉末来制取粉末烧结多孔材料。探讨了粉末形状及松装密度对不锈钢粉末烧结多孔材料制造工艺中的成形压力和烧结温度等工艺参数的影响;研究了原料粉末松装密度对不锈钢粉末烧结多孔材料的透气性、拉伸强度的影响。结果表明:成形压力、烧结温度和制品的透气性受粉末松装密度影响显著。粒度范围为0.18～0.90mm时,气雾化粉末的成形压力比水雾化粉末要高近1倍;当粉末的粒度相同时,采用松装密度大的球形粉末所需的烧结温度比松装密度小的不规则粉末的高60～70℃;粒度为0.45～0.60mm时,选用松装密度为4.13 g/cm3粉末所制备的多孔制品的透气性为3.16×10-10m2,而选用松装密度为2.67 g/cm3的粉末所制备的多孔制品的透气性仅为8.8×10-11m2。不锈钢多孔材料的强度受原料粉末的松装密度影响显著;粒度相同,制备工艺相同时,采用较低松装密度的粉末的制品,能够得到较高的强度。     

Elevated temperature mechanical properties of shock-strengthened austenitic stainless steel

The mechanical properties of explosively shock-strenghened (320 kbar) Type 304 stain-less steel were investigated. Tensile tests were performed on the shock-strengthened material at temperatures ranging from room temperature to 650°C. The shock hardening caused large increases in yield strength, moderate increases in ultimate strength, and reductions in ductility. Annealing studies on the shock-strengthened stainless steel indicated that considerable amount of the shock-induced strength stainless steel indicated that considerable amount of the shock-induced strength increase is retained after long time anneals at 700 and 750°C. However, metallographic studies on the annealed material indicated the presence of fine carbide particles which may partly account for the strength of the material. Creep studies indicated that the minimum creep rate at 650°C was reduced by a factor of 1000 due to shock strengthening.     

304不锈钢表面冷喷涂TC4钛合金涂层性能研究

采用冷喷涂技术在304不锈钢表面制备了TC4钛合金涂层,通过扫描电子显微镜观察了涂层的形貌、组织结构,并利用电化学方法研究了涂层的腐蚀电化学特征。研究结果表明,冷喷涂制备的TC4钛合金涂层致密性存在较为明显的梯度现象,靠近基体的涂层密度明显高于表面;涂层喷涂过程没有出现明显氧化现象,与基体的结合强度可达20 MPa左右;涂层的耐腐蚀性能优于304不锈钢,可大大提升不锈钢材料在海洋环境中的耐点蚀性能。     

Temperature and strain rate dependence of cyclic deformation response and damage accumulation in ofhc copper and 304 stainless steel

The effects of temperature and strain rate on deformation behavior and dislocation structure were investigated for OFHC copper and type 304 stainless steel. It is shown that the cyclic stress response is inversely related to the cell size for copper cycled at different temperatures ranging from -75 to 650°C. Type 304 stainless steel underwent a change from a planar to a wavy slip character as the temperature was changed from room temperature to 760°C. At elevated temperatures, cells were observed and the size of the cells tended to increase with increase in temperature. The effects of temperature on the cyclic stress-strain parameters were investigated for copper, type 304 stainless steel and Ferrovac “E” iron. On studying the effects of temperature and strain rate on the fracture mechanisms it was found that a time dependent fracture mode was dominant at high temperature levels and low strain rates. However, at high strain rates the life was insensitive to temperature. The role of grain boundary migration on the fracture process was investigated. Grain boundary migration was found to be dependent on strain rate for copper. However, for type 304 stainless steel, the grain boundary migration was inhibited at high temperature (760°C) due to the presence of precipitates at the grain boundaries. In strain cycling of OFHC copper and type 304 stainless steel, it was found that the addition of creep-type damage to fatigue damage resulted in a total damage which was not equal to unity for failure when these different modes were imposed sequentially. The sense of the damage accumulation appeared to have no effect on this summation.     

Structure property correlations in hot isostatically pressed AISI-304 stainless steel

Abstract

Stainless steel components are usually fabricated by conventional manufacturing methods. This route becomes uneconomical for highly intricate shapes. Therefore, processing of AISI-304 stainless steel has been carried out by powder metallurgy and hot isostatic pressing (HIP) route. The microstructure and mechanical properties of the hipped steel produced from inert gas atomised powder were studied. The steel has shown a homogeneous and fine grained microstructure containing annealing twins and the absence of undesirable prior particle boundaries. This has led to higher yield strength, ultimate tensile strength and the ductility than that of the conventionally processed steel. The tensile deformation behaviour of hipped steel was found to be similar to that of the wrought material. Impact strength and fracture toughness of the hipped steel were superior to that of the wrought material; hence the hipped steel could be recommended for manufacture of components for critical applications.     

High corrosion resistance 21%Cr-0.4%Cu ferritic stainless steel contributing to resource conservation

With prices for metal resources such as nickel and molybdenum soaring,there is a heightened sense of crisis concerning resource scarcity.While Type304,the most common stainless steel,offers excellent corrosion resistance,its price is affected significantly by the cost of nickel because of its 8%nickel content.The stainless steel that has the same corrosion resistance as that of Type304 and does not contain nickel and molybdenum has been required.JFE Steel Corporation has developed a new 21%Cr-0.4%Cu stainless steel,the world’s first ferritic stainless steel,which offers equivalent corrosion resistance to Type304 while containing absolutely no nickel or molybdenum,two rare metals.The newly developed steel contains 21%chromium with the addition of 0.4% copper.The development of the steel is based on a new discovery that the passive films of stainless steels could be strengthened by the synergy effect of high chromium content and copper addition.Copper addition enriches the chromium content in passive films after field exposure.Newly developed 21%Cr-0.4%Cu stainless steel is adopted for many applications as a substitution for Type304,including commercial kitchenware,building materials and industrial machinery.The steel is expected to be a new standard of a ferritic stainless steel as a substitution for Type304.     

Effects of prior cold working on low cycle fatigue behavior of stainless steels, titanium alloy timetal 834 and superalloy IN 718: A review

This paper gives a brief review of the efforts made to study the effects of cold rolling on low cycle fatigue (LCF) behavior of stainless steels, titanium alloy Timetal 834 and Ni-Fe based Super alloy IN 718 at different temperatures and different strain amplitudes. Low Cycle Fatigue tests on cold worked 316L stainless steel at various temperatures from room temperature to 923K have been reported. In all tests the effect of 20% prior cold work(PCW) on LCF behavior of type 316L (N) stainless steel had been studied at 873K under total axial strain controlled mode in air at strain amplitudes from ±0.25% to ±1.0%. Fatigue life in the solution annealed condition was similar to that of the PCW material at higher strain amplitudes (≥0.5%) while at lower strain amplitudes the PCW material exhibited longer life. The influence of PCW on LCF behavior of type 304 and AISI 304LN stainless steel was studied at various temperatures and it was observed that the fatigue life was strongly dependent on prior cold work, temperature, and strain amplitude employed. Cold rolling of the titanium alloy Timetal 834 and age hardenable Ni-Fe-based superalloy IN 718 has been found to cause marked enhancement in LCF life at low strain amplitude and eliminate the bilinearity from the Coffin-Manson(C-M) relationship. Work carried on this aspect, however is very limited particularly in the case of Titanium alloys and Ni-Fe based Superalloy IN 718     

Experimental investigation on cyclic deformation behaviour of primary heat transport piping materials: Masing analysis of hysteresis loops

In this work, the hysteresis loops of 304LN stainless steel at various strain amplitudes obtained via multi specimen, incremental and decremental load schemes were analysed for their Masing behaviour. In all the loading schemes, the material showed a non-Masing behaviour. A probabilistic density approach is used to examine the variation of yield during cyclic deformation. It is noted that yield level increased with increasing strain amplitude in all the test methods. The change in yield level is attributed to the microstructural changes in the material. Variation in microstructure and yield are thought to be responsible for the non-Masing behaviour of 304LN stainless steel.     

3D打印用金属粉末的制备研究

针对国内3D打印用金属粉体材料的现状,通过对雾化喷嘴的设计改造,采用气雾化方法制备了304L不锈钢粉末;并通过扫描电镜、激光粒度测量仪以及霍尔流量计等对粉末的颗粒形貌、粒度分布、流动性等进行了观察研究。结果表明:采用自制双层雾化喷嘴制备的合金粉末,球形度高,粒度分布范围窄,粉末平均粒径为40μm,符合3D打印对金属粉体材料的要求。     

The effects of cold working on sensitization and intergranular corrosion behavior of AISI 304 stainless steel

The effects of prior cold rolling of up to an 80 pct reduction in thickness on the sensitization-desensitization behavior of Type AISI 304 stainless steel and its susceptibility to intergranular corrosion have been studied by electrochemical potentiokinetic reactivation (EPR) and Strauss-test methods. The results indicate that the prior deformation accelerated the sensitization as compared to the undeformed stainless steel. The deformed Type 304 stainless steel experienced desensitization at higher temperatures and times, and it was found to be enhanced by increased cold deformation. This could be attributed to the increased long-range chromium diffusion, possibly brought on by increasing pipe diffusion and vacancies. The role of the deformation-induced martensite (DIM) and texture, introduced by uniaxial cold rolling, on the sensitization-desensitization kinetics has also been discussed. This study could not reveal any systematic relationship between texture and the degree of sensitization (DOS) obtained. The effect of DIM on DOS seems to be pronounced at 500 °C when the steel retained significant amounts of DIM; however, the retained DIM is insignificant at higher sensitization times and temperatures.     

An Assessment of the Ductile Fracture Behavior of Hot Isostatically Pressed and Forged 304L Stainless Steel

Type 300 austenitic stainless steel manufactured by hot isostatic pressing (HIP) has recently been shown to exhibit subtly different fracture behavior from that of equivalent graded forged steel, whereby the oxygen remaining in the component after HIP manifests itself in the austenite matrix as nonmetallic oxide inclusions. These inclusions facilitate fracture by acting as nucleation sites for the initiation, growth, and coalescence of microvoids in the plastically deforming austenite matrix. Here, we perform analyses based on the Rice–Tracey (RT) void growth model, supported by instrumented Charpy and J-integral fracture toughness testing at ambient temperature, to characterize the degree of void growth ahead of both a V-notch and crack in 304L stainless steel. We show that the hot isostatically pressed (HIP’d) 304L steel exhibits a lower critical void growth at the onset of fracture than that observed in forged 304L steel, which ultimately results in HIP’d steel exhibiting lower fracture toughness at initiation and impact toughness. Although the reduction in toughness of HIP’d steel is not detrimental to its use, due to the steel’s sufficiently high toughness, the study does indicate that HIP’d and forged 304L steel behave as subtly different materials at a microstructural level with respect to their fracture behavior.     

Prediction of fatigue crack formation in 304 stainless steel

Strain-controlled fatigue tests have been conducted on center-holed 304 stainless steel specimens. The fraction of total fatigue life spent until formation of an “engineering” crack ranged from about 15 to 85 pct, indicating the potential importance of being able to predict the fatigue crack formation life. A “just formed engineering crack,” as defined here, is a through crack long in the thickness direction, which has just emerged from the center hole. An energy based parameter, ΔσrΔε,, has been shown to correlate with the appearance of fatigue cracks in the center-holed 304 stainless steel specimens. This parameter is suggested to be more useful in predicting fatigue crack formation life than Δσ or Δε, alone. A good correlation was found over the limited range of data for two types of 304 stainless steel, a powder metallurgy (PM) stainless steel with higher than normal strength prop-erties and an ingot metallurgy (IM) stainless steel with normal strength properties. A better correlation was found for strain-controlled fatigue tests which did not go into compressive strain than for com-pletely reversed fatigue. Formerly a graduate student with the Materials Science and Engineering Department, Northwestern University, is     

Correlation between precipitation reactions and bulk density changes in type 18-12 austenitic stainless steels

Dimensional changes in austenitic stainless steels exposed to fast neutron fluxes have been attributed mainly to void formation and thermal and irradiation induced creep. In this in-vestigation, immersion bulk density measurements were performed on variously preheated AISI Types 304, 316L and 316 stainless steels. The results show that the density changes accompanying sequential precipitation of the various carbide (M₂₃C₈, M₆C) and intermetallic phases (sigma, chi, eta) during thermal aging can be comparable to those attributed to void formation at low fast neutron fluences. Vacuum melted Type 316 stainless steel always den-sified with aging at 725°C or 810°C and exhibited higher rates of precipitation and recovery of cold work during aging than the equivalent air melted steel. The direction of the density change in the air melted material was dependent upon the prior thermomechanical treat-ments. This behavior can be rationalized on the basis of a higher impurity content in the air melted material. A mathematical model was constructed which successfully explains the variation of bulk density due to sequential precipitation in Type 18-12 austenitic stain-less steels.     

TTS443高铬铁素体不锈钢的开发

针对304(Crl8-Ni8型)奥氏体不锈钢和430(Cr17型)铁素体不锈钢的特性,通过试验和分析Cr、Cu、Nb、Ti等合金元素对铁基合金材料性能的影响,开发出一种高铬铁素体不锈钢-TTS443(/%:O.010C、21Cr、0.40Cu、0.25Nb、0.20Ti、O.012N) 。该钢种的耐蚀性能与304奥氏体不锈钢相当,具有良好的成形性与焊接性能,TTS443铁素体不锈钢是304奥氏体不锈钢理想代替材料。     

Elastic Modulus of 304 Stainless Steel Coating by Cold Gas Dynamic Spraying

 304 stainless steel coating was deposited on the IF steel substrate by cold gas dynamic spraying (CGDS), and the elastic modulus of the 304 stainless steel coating was studied. The elastic modulus of cold sprayed 304 stainless steel coating was measured using the three-point bend testing and the compound beam theory, and the other mechanic parameters (such as the equivalent flexural rigidity and the moment of inertia of area) of the coatings were also calculated using this compound beam theory. It is found that the calculated results using the above methods are accurate and reliable. The elastic modulus value of the cold sprayed 304 stainless steel coating is 1179×105 MPa, and it is slightly lower than the 304 stainless steel plate (about 2×105 MPa). It indicates that the elastic modulus of the cold sprayed coatings was quite different from the comparable bulk materials. The main reason is that the pores and other defects are existed in the coatings, and the elastic modulus of the coatings also depends on varies parameters such as the feed stock particle size, porosity, and processing parameters.     

304不锈钢精密钢带表面“炉灰”缺陷分析

 在304不锈钢精密带光亮退火生产环节，钢带表面易出现金属粉的现象，导致产品表面质量存在不稳定性。针对304不锈钢精密钢带表面出现的“炉灰”缺陷，利用扫描电子显微镜观察其形貌，并用能谱仪对缺陷处局部成分进行了点扫描和面扫描检测。结果表明，304精密钢带“炉灰”缺陷微观形貌为白色微颗粒。结合能谱微区分析结果和相关研究分析，推测“炉灰”缺陷主要是由于304不锈钢基体组织中硼元素质量分数过高，钢带在光亮退火过程中硼原子易与保护气氛中分解的活性较高的氮原子结合，生成氮化硼析出而形成的。通过控制不锈钢基体中硼元素质量分数不大于0.001 5%，从而达到有效降低304不锈钢精密钢带“炉灰”缺陷的目的。     

Cyclic creep of type 304 stainless steel during unbalanced tension-compression loading at elevated temperature

Samples of Type 304 stainless steel were subjected to cyclic stresses with a positive mean stress at 300 and 560°C. Very rapid net elongation was observed whenever the stress limits were such as to produce a plastic strain amplitude of the same order of magnitude as the elastic strain at the peak stress. The maximum mean strain-rate, or cyclic creep rate, for a given peak tensile stress was achieved when the mean stress was just slightly above zero. Increasing the mean stress caused the mean strain rate to de-crease. The sensitive dependence of the mean strain-rate on the plastic strain ampli-tude and inverse dependence on the mean stress indicates that remobilization of disloca-tions by the reverse strain is an important mechanism for cyclic-creep acceleration. Although rapid cyclic creep was observed at both temperatures, a measurable mean strain rate was found for a much narrower range of stress conditions at 560 than at 300°C. The strain accumulated during cyclic creep did not produce any strain hardening, but did influence the shape of the stress-strain curve in a subsequent tensile test.     

碰撞速度对冷喷涂粒子沉积行为影响的数值模拟研究

采用有限元数值模拟方法研究了冷喷涂过程中304不锈钢粒子的碰撞速度对IF钢基板的碰撞和沉积变形行为的影响。结果表明,当粒子速度高于一个在产生绝热失稳变形并发生沉积的临界速度时,粒子的扁平率、有效接触界面积均显著增加。粒子沉积过程中的剪切失稳行为是由撞击过程中产生的压力与摩擦力共同作用的结果。这个临界速度可以由数值模拟计算得到,用于优化多种材料的冷喷涂工艺参数。     

金属粉末激光烧结过程中搭接率敏感性分析

依据金属粉末直接激光烧结(DMLS)的实际烧结过程,以304不锈钢粉作为烧结材料,利用Ansys有限元软件,对其烧结成形温度场进行了数值模拟。首先对多道烧结过程中温度场的变化规律进行了分析,进而分析不同搭接率对温度场的影响,得出搭接率对烧结效果的敏感性。结果表明:搭接率过低会导致相邻烧结道之间存在未烧结粉末区域;搭接率过高又会造成温度过量集中,热量积累严重。针对所研究的304不锈钢粉,搭接率在26%~33%之间时,烧结效果较好。