Effects of deformation-induced martensite and grain size on ductile-to-brittle transition behavior of austenitic 18Cr-10Mn-N stainless steels

Effects of deformation-induced martensite and grain size on ductile-to-brittle transition behavior of austenitic 18Cr-10Mn-(0.3∼0.6)N stainless steels with different alloying elements were investigated by means of Charpy impact tests and microstructural analyses. The steels all exhibited ductile-to-brittle transition behavior due to unusual brittle fracture at low temperatures despite having a face-centered cubic structure. The ductileto-brittle transition temperature (DBTT) obtained from Chapry impact tests did not coincide with that predicted by an empirical equation depending on N content in austenitic Cr-Mn-N stainless steels. Furthermore, a decrease of grain size was not effective in terms of lowering DBTT. Electron back-scattered diffraction and transmission electron microscopy analyses of the cross-sectional area of the fracture surface showed that some austenites with lower stability could be transformed to α’-martensite by localized plastic deformation near the fracture surface. Based on these results, it was suggested that when austenitic 18Cr-10Mn-N stainless steels have limited Ni, Mo, and N content, the deterioration of austenite stability promotes the formation of deformation-induced martensite and thus increases DBTT by substantially decreasing low-temperature toughness.     

Thermal Effects That Arise upon Different Heat Treatments in Austenitic Steels Alloyed with Titanium and Phosphorus

Structural and microstructural changes that arise in the course of the heat treatment of Cr–Ni–Mo austenitic stainless steels with different concentrations of titanium and phosphorus have been studied. It has been found that the alloying with phosphorus decreases the lattice parameter of these steels. The phosphorus contribution to this effect is 0.015 ± 0.002 Å/at %. Aging at a temperature of 670 K for about 20 h leads to the precipitation of dispersed needle-like particles, which are most likely to be iron phosphides. In the temperature range of 700–800 K, in austenitic steels, the atomic separation of the solid solution occurs, the intensity of which decreases upon alloying with titanium or phosphorus at concentrations of 1.0 and 0.1 wt %, respectively. At higher temperatures (about 950 K), the formed precipitates of the Ni₃Ti (γ') phase increase in size to 7–10 nm.     

Chloride-induced stress corrosion cracking of Duplex stainless steels. Models,test methods and experience

Stress corrosion cracking (SCC) induced by chlorides frequently causes problems in applications where standard austenitic stainless steels are being used. Often this problem can be solved by the use of duplex stainless steels. In this report the mechanisms for SCC have been surveyed, and the cause for the high SCC resistance of duplex stainless steels has been discussed and evaluation of test methods for SCC and how duplex stainless steels respond to them, as well as practical experience of duplex stainless steels. The study shows that no single mechanism can be attributed to the good resistance to SCC of duplex stainless steels. Probably a synergistic effect of electrochemical and/or mechanical effects is responsible for the good performance. Test methods for SCC often give relatively good correspondence with real applications, but ranking is often doubtful, and comparisons of different material types should be made with caution. Numerous cases with SCC on standard austenitic stainless steels have been solved by the use of duplex stainless steels.     

Localized corrosion of (lean) duplex stainless steels in immersion units of urban wastewater treatment plants

With lower alloying cost and higher mechanical properties, lean duplex stainless steels can be an alternative to the more commonly used austenitic stainless steels. However, these alloys are still not the preferred choice, probably due to a lack of field experience. A study was thus initiated in view of defining the limits of use of selected (lean) duplexes for urban wastewater treatment units. The present paper shows the localized corrosion performance of selected lean duplexes in chloride contaminated solutions. The results are compared with austenitic S30403 and S31603 and with the more standard duplexes S82441 and S32205. The effect of welding was also investigated. Exposures in field municipal wastewater plants were conducted for 1 year in low and high chloride content units. The results show that lean duplexes S32101 and S32202 can be used as alternatives to S30403 and S31603 in low chloride electrolytes. At 500 ppm of chloride content, duplex stainless steel S32304 showed better corrosion resistance than S30403 and S31603. For higher chloride contents (1000 ppm and above) the standard duplexes S82441 and S32205 shall be preferred.     

Corrosion behavior of martensitic and precipitation hardening stainless steels treated by plasma nitriding

Plasma nitriding is a well established technology to improve wear and corrosion properties of austenitic stainless steels. Nevertheless, in the case of martensitic stainless steels, it continues being a problem mainly from the corrosion resistance viewpoint.In this work, three high chromium stainless steels (M340, N695 and Corrax) were hardened by ion nitriding at low temperature, intending to preserve their corrosion resistance.Corrosion behavior was evaluated by CuSO₄ spot, salt spray fog and potentiodynamic polarization in NaCl solution. Microstructure was analyzed by optical microscopy, SEM (EDS) and glancing angle X-ray diffraction. All the samples showed an acceptable corrosion resistance in experiments with CuSO₄, but in salt spray fog and electrochemical tests, only Corrax showed good behavior. The poor corrosion performance could be explained by chromium carbides formed in thermal treatment stage in martensitic steels and chromium nitrides formed during nitriding, even though the process was carried out at low temperature.     

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.     

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.     

Temperatur- und Konzentrationsabhängigkeit der Korrosion nichtrostender austenitischer und ferritischer Werkstoffe in 20- bis 75%iger Salpetersäure

Corrosion resistance of austenitic and ferritic stainless alloys in 20 to 75% nitric acid as a function of temperature and concentration A series of stainless austenitic and ferritic materials was exposed for 100 days to boiling nitric acid which contained no corrosion products; the corrosion rates and depths of the grain boundary attack were observed. Provided the structure is precipitation-free, the following are suitable for long-term exposure; the austenitic steels X 2 CrNi 1912, X1 CrNi 25 21, X1 CrNiMoN 25 222 and X1 NiCrMoCu31274, the practically Mo-free and Cu-free development steel X1 NiCr31 27, and the highly Mo-alloyed variant X1 NiCrMoCu 31275. In the case of alloy NiCr21 Mo it is advisable to limit the concentration and/or the temperature of the nitric acid. The “superferrite” X1CrNiMoNb2842, the Japanese steel X1 CrNiNb 30 2 and the austenitic steels X2 CrNiMoN 1713 3 and X1 CrNiMoN 25 22 2 in the version with high nickel content are unsuitable. Thus, as an alloying element, molybdenum does not always impair the resistance of stainless steels to nitric acid. The decisive factor affecting the corrosion rates is the chromium content of the material. The temperature-dependent function of the corrosion in azeotropic nitric acid conforms to Arrhenius relations. The concentration-dependent function of the corrosion in 20 to 75 (80)% nitric acid can be described by a hyperbolic equation. An exception is formed by X1 CrNiSi 1815; here the corrosion rate increases with the concentration of the acid until the azeotropic point is reached; then, owing to the formation of a surface film, it falls until the acid becomes highly concentrated.     

典型不锈钢晶间腐蚀敏化温度的研究

用电化学动电位再活化（EPR）法、硫酸—硫酸铜法及扫描电镜研究了典型的202、304奥氏体不锈钢与409、430铁素体不锈钢在不同敏化温度下晶间腐蚀的敏感性。结果表明,奥氏体与铁素体不锈钢敏感温度区间不同,奥氏体不锈钢诱发晶间腐蚀的敏感温度约为650℃,铁素体不锈钢诱发晶间腐蚀的敏感温度约为950℃。研究结果为正确地评判不锈钢晶间腐蚀敏感性及优化生产工艺提供了科学依据。     

Formation of Si diffusion layer on stainless steels and their high temperature corrosion resistance in molten salt

The surface alloying of Si into SUS430 and SUS304 stainless steels was achieved by a pack-cementation method. A high-temperature corrosion test was undertaken under a corrosive ash in a high-temperature gas, which simulated the environment of a waste incinerator. The effect of the surface alloying of Si on the high temperature corrosion resistance of these stainless steels was examined. The result of the corrosion test showed that the corrosion mass loss of the siliconized stainless steel was lower than that of non-treated stainless steel. It was thought that the high temperature corrosion resistance in the corrosive ash containing large amounts of molten salt was improved by the surface alloying of Si. The change in corrosion loss with test time in the corrosive ash was also examined. The corrosion loss of the non-treated stainless steels rapidly increased. On the other hand, the corrosion rate of the siliconized stainless steels was small.It was found from the observation and EPMA analysis of a cross-section of the specimen after the corrosion test that, for the non-treated stainless steels, the metallic elements were dissolved into the molten salt. On the other hand, for the siliconized stainless steels, a scale consisting of silicon oxide was formed on the steels. As a result, the dissolved metallic elements were small.The electrochemical studies indicated that there was an effect of the surface alloying of Si on the hot corrosion resistance of the stainless steels. Hence, the corrosion potentials of the siliconized steels were higher than those of the non-treated steels. The increase in the anodic current densities for the siliconized steels was smaller than that for the non-treated steels. It was thought that the electrochemical corrosion reaction was difficult to occur due to the surface alloying of Si into the stainless steels.     

再生器中奥氏体不锈钢取热盘管失效原因分析

分析表明炼油厂催化裂化装置再生器取热盘管失效的主要原因是腐蚀疲劳。本文就该实际工程条件下,对1Cr18Ni9Ti及1Cr25Ni20两类奥氏体不锈钢在高温含微量氯离子水中的腐蚀疲劳现象及成因进行了讨论。改善汽水流动条件可显著延长盘管的使用寿命。     

Corrosion resistance of low‐nickel duplex stainless steel rebars

The use of stainless steel bars in reinforced concrete structures may be an effective method to prevent corrosion in aggressive environments where high amounts of chlorides may penetrate in the concrete cover. For an estimation of the service life of structures where stainless steel bars are used, the chloride threshold for these rebars should be defined, and the influence of chemical composition and metallurgical factors that may affect the corrosion resistance (strengthening, welding, etc.) should be assessed. To reduce the cost of stainless steel reinforcement, duplex stainless steels with low nickel content have been recently proposed as an alternative to traditional austenitic steels, even though, few results are available regarding their corrosion performance in chloride contaminated concrete. This paper deals with the corrosion resistance of low‐nickel duplex stainless steel rebars (1.4362 and 1.4162) as a function of the chloride content. Comparison is made with traditional austenitic steels. An attempt to define a chloride threshold for the different stainless steels is made by comparing the results of several test procedures both in concrete and in solution.     

445铁素体不锈钢内胆水箱的加工性能和耐蚀性

将445铁素体不锈钢的主要化学成分、力学性能、成型性能和焊接性能等基本性能与304奥氏体不锈钢进行对比,结果表明,445不锈钢具有较好的机加工性能。采用盐雾试验及10%的NaCl溶液加速腐蚀试验等方法,对比445水箱、304水箱及两者混合搭配的内胆水箱的太阳能热水器的耐腐蚀性能。结果表明,445不锈钢耐腐蚀性稍逊于304不锈钢,在80～120℃时,445与304不锈钢均发生蒸汽腐蚀、水线腐蚀,且445不锈钢出现较为严重的点蚀现象。     

Structural steels for cores of fast neutron reactors

The directions of development of stainless steels for fuel rod claddings and fuel element cans of fast neutron reactors are considered. The principles of complex alloying and treatment of steels ensuring low swelling at irradiation dozses of 80–90 dpa (dislocations per atom) are described. Ways for further development of functional characteristics of steels of austenitic and martensitic classes are outlined.     

High-temperature oxidation and thermal cycling of aluminum-electroplated stainless steels

An alumina coating, produced from the oxidation of an aluminum-electroplated deposit, improved the oxidation resistance in air of a ferritic, AISI-type 446 stainless steel, Fe-24Cr-1.2Al containing 0.15% of mischmetal, and an austenitic AISI 321 stainless steel containing 0.53% Ti, at least up to 1100°C. In thermal-cycling tests from 1000°C to room temperature, the alumina coating was adherent on the ferritic and austenitic steels, for at least 1000 and about 700 cycles, respectively. The addition of rare earths to the ferritic steels and titanium to the austenitic, provided good adhesion between the coating and substrate. The porous nature of the coating was found to be very beneficial by causing the coating to be more resistant to thermal and growth stresses. Oxidation mechanisms are discussed in the light of results obtained from the thermogravimetric tests and metallographic observations by SEM-ED analysis.     

The correlation between mechanical and electrochemical parameters of stress corrosion cracking of austenitic stainless steels

Extension measurements of exposed specimens of austenitic stainless steels in hot magnesium chloride solutions are interpreted with the potential-time curves. For austenitic stainless steels, it is very difficult to determine the yield point; it is necessary to study the stress-elongation curves at different elongation rates and to known the creep behaviour of the steels. In dead load stress corrosion tests the elongation-time curves allow the incubation time of stress corrosion cracks to be distinguished from the propagation time. The propagation time is more important than incubation time for the classification of the susceptibility of austenitic stainless steels to stress corrosion cracking, because the incubation time is more dependent on experimental procedure than is the propagation time. The stainless steel classification obtained was compared with a new test in which a load is applied and immediately taken off; after a rapid fall the potential-time behaviour provides information about the crack velocity.     

高氮奥氏体不锈钢研究进展

目前高氮钢研究的主要热点是高氮不锈钢,而高氮奥氏体不锈钢的应用前景最被看好.综述近年来国内外高氮奥氏体不锈钢的研究现状,包括氮在奥氏体不锈钢中的作用机理;高氮奥氏体不锈钢的试制;高含量氮对奥氏体不锈钢力学性能、耐蚀性能和组织稳定性的影响以及对高氮不锈钢应用前景的展望.     

Structural steels for cores of fast neutron reactors

The directions of development of stainless steels for fuel rod claddings and fuel element cans of fast neutron reactors are considered. The principles of complex alloying and treatment of steels ensuring low swelling at irradiation dozses of 80–90 dpa (dislocations per atom) are described. Ways for further development of functional characteristics of steels of austenitic and martensitic classes are outlined.     

Thermomechanically treated type 18-10 steels: Structure, properties, and corrosion resistance

The strength characteristics of stainless steels can be enhanced by a form of alloying (in particular, with nitrogen) that hardens the solid solution, by dispersion hardening, and by the formation of an austenitic-ferritic or martensitic structure. All of such methods, except solid-solution hardening, entail the formation of a heterogeneous structure and will sometimes reduce the corrosion resistance of the steel. On the other hand, austenitic stainless steel can be strengthened without polymorphic transformations by thermomechanical treatment (TMT). As applied to stainless steels, this involves controlled rolling with precisely specified temperature range, strain rate, amount of strain, and cooling rate at the end of deformation. This study was concerned with the influence of various TMT schedules on the mechanical properties, corrosion resistance, structure, and weldability of 08Kh18N10T and 03Kh18N11 steels produced in the laboratory and commercially. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 13–19, October, 1997.     

无镍高氮奥氏体不锈钢的脆韧转变

研究了无镍高氮奥氏体不锈钢的脆韧转变(BDT)。在176 K、273 K和336 K进行的落锤试验结果表明,尽管Fe-25Cr-1.1N(质量分数,%)是面心立方结构的奥氏体合金钢,但仍展现出显著的脆韧转变现象。对冲击试验试样的塑性变形观察表明,BDT是由于低温下差的延展性所致,这与铁素体钢的情况是一致的。为了测量BDT的激活能,利用4点弯曲试验研究了应变速率与BDT温度的关系。研究发现,BDT温度与应变速率之间的依赖关系不显著,且BDT温度对应变率的Arrhenius曲线表明Fe-25Cr-1.1N钢BDT的激活能比低碳铁素体钢的高得多。从滑移位错与溶质氮原子发生交互作用导致低温下位错可动性降低这一角度,本文探讨了高氮钢特有的BDT及其高激活能的本质原因。