Austenitic stainless steels at cryogenic temperatures 1—Structural stability and magnetic properties

The structure and magnetic properties of some 15 austenitic stainless steels were examined after cyclic cooling treatments and low temperature deformation. Magnetic measurements at room temperature, 77 K, and 4.2 K and subsequent metallographic examination suggest that many of the AISI 300 stainless steels such as 301, 302, 303, 304, 304L, 305, 316L, 321, and 347 must be considered potentially unstable with respect to the formation of the ferromagnetic α′ martensite phase on repeated cooling to low temperatures. This structural instability was increased significantly after a sensitizing treatment in the weldable steels 304L, 321, and 347 leading to the formation of up to 11.2% a′ martensite, part of which formed isothermally. Low temperature deformation is even more potent in promoting the transformation, at least 50% α′ martensite being induced by deformation at 4.2 K in the otherwise stable alloys such as 309 and the 0.2% N versions of 304L and 316L. The high alloy steels 310 and Kromarc 55 remain fully a austenitic even after deformation to rupture at 4.2 K. The temperature dependence of the magnetic susceptibility of the latter alloys and Incoloy 800 indicates that their low temperature structural stability is associated with magnetic transitions which occur within the austenite phase.     

Corrosion susceptibilities of various metals and alloys in synthetic geothermal brines

The corrosion susceptibilities of various pure metals and alloys were investigated in synthetic geothermal fluids. Rates of corrosion of AISI 1010 steel, types 304 and 316 stainless steels, Monel 400 and nickel were determined at three temperatures (296, 333 and 368 K); and those of the molybdenum, niobium and titanium were determined at 368 K only. Type 304 stainless steel appears to undergo an active-passive transition at a temperature range between 333 and 368 K. In the passive state type 304 steel has essentially the same corrosion rate as type 316. At 368 K the corrosion rate of pure nickel was approximately 2.5 times that of Monel, which in turn was twice that of type 316 stainless steel. The corrosion rates of Mo, Nb and Ti were less than one mdd at the highest experimental temperature.     

Hot cracking susceptibility of boron modified AISI 304 austenitic stainless steel welds

Abstract

The hot cracking susceptibility of welds made on AISI 304 stainless steel modified with from 0·2 to 1·0 wt-%B has been investigated. Varestraint tests showed that the hot cracking susceptibility is high for boron additions of about 0·2%, but is decreased when the boron content is increased to ≥0·5%. Steels containing about 0·2%B were found to have a wide solidification temperature range and their high temperature ductility was low compared with boron free AISI 304 steel and the other boron modified steels. Ferrite precipitation was inhibited in the 0·2%B steels and the formation of low melting point grain boundary films was thereby promoted. Increasing the boron content to ≥0·6% reduces the coefficient of thermal expansion and narrows the solidification temperature range. In addition, crack refilling was observed, resulting in improved hot ductility and high resistance to hot cracking. It is concluded that in structures where weld restraint forces are not high, hot cracking is not likely to occur if boron additions of >0·6% are made to AISI 304 stainless steel. In T-type and Fisco weld cracking tests, in which the weld restraint forces are close to those experienced by actual structural welds, the boron modified stainless steels show a low hot cracking susceptibility which is not significantly different from that of boron free AISI 304 steel.

MST/1548     

Fabrication and characteristics of porous AISI 304L stainless steels by ceramic powder additions

Abstract

Porous AISI 304L stainless steels were fabricated by a new powder metallurgy technique, based on the addition of oxide based ceramic powders. The mixture of AISI 304L stainless steel powders and oxide based ceramic powders was compacted using a hand press at a pressure of 294 or 490 MPa. The green compacts were sintered at 1150 or 1200°C for 3 h in Ar gas atmosphere. The addition of oxide based ceramic powders into AISI 304L stainless steel powders gave rise to porous AISI 304L stainless steels with fine pores. Also, the addition of the ceramic powders increased the hardness.     

Sensitization evaluation of the austenitic stainless steel AISI 304L, 316L, 321 and 347

This work presents a systematic investigation of the influence of time and temperature in the sensitization of stainless steel AISI 304L, AISI 316L, AISI 321 and AISI 347 pipes used in petroleum refining plants. The sensitization was assessed by Scanning Electron Microscopy (SEM) according to ASTM A-262 and by the Double Loop Electrochemical Potentiokinetic Reactivation test (DLEPR). The results showed that all steels did not present sensitization at operating temperature (380°C) in the desulfurizers process, but the temperature of 500°C was critical to the appearing of sensitization for the both low carbon stainless steels and AISI 321 SS, while for the AISI 347 the critical temperature was 550°C. The stabilized steels confirmed to be more resistant to sensitization than the low carbon stainless steels, and niobium showed to be more efficient stabilizing agent than titanium.     

Investigation of the mechanical properties of friction-welded joints between AISI 304L and AISI 4340 steel as a function rotational speed

In this study, standard AISI 304L austenitic stainless steel and AISI 4340 steel couple were welded by friction welding process using five different rotational speeds. The joining performances of AISI 304L/AISI 4340 friction-welded joints were studied and the influences of rotational speed on the microstructure and mechanical properties of the welded joints were also estimated. The microstructural properties of heat affected zone (HAZ) were examined by scanning electron microscopy (SEM). The microhardness across the interface perpendicular to the interface was measured and the strength of the joints was determined with tensile tests. The experimental results indicate that the tensile strength of friction-welded 304L/4340 components were markedly affected by joining rotational speed selected.     

Improved mechanical properties in dissimilar Ti-AISI 304L joints

The effect of low temperature post weld heat treatment on the tensile strength and bend test properties of dissimilar friction welds between titanium and AISI 304L stainless steel joints is investigated. Post weld heat treatment at temperatures less than 873 K has no effect on joint tensile strength properties, but markedly improves bend test properties. The highest bend angle is produced using a post-weld heat treatment at 773 K for 1 h (the Larson-Miller parameter corresponding to this treatment is 15.5×10³ K h^–1). Low temperature heat treatment improves bend ductility, because stress relaxation occurs with minimal increase in the transition region width at the bondline region. Dissimilar joint bend testing properties decrease markedly when the width of the transition region exceeds 1–2 m. An explanation for the detrimental effect of thick transition regions at the joint interface region on the mechanical properties of dissimilar joints is proposed. It is suggested that the development of significant triaxial stress due to the constraint imposed by large, needle-shaped intermetallic particles promotes premature joint failure in joints containing thick transition regions.     

The sensitivity of surface crack initiation to surface roughness in low-cycle fatigue at high temperature

Low-cycle fatigue tests have been carried out on AISI 304L stainless steel and Cr---Mo---V steel specimens with two different modes of surface roughness at 823 K. In the case of Cr---Mo---V steel, grain boundary cavities were not formed during the test. Transgranular cracks were formed and then propagated. The number of cycles required for the crack initiation was observed to be a very large fraction of the toral fatigue life. In the case of AISI 304L stainless steel, grain boundary cavities formed and intergranular crack initiation and propagation was also observed to occur. The number of cycles required for crack initiation was negligible in comparison with the total low-cycle fatigue life.     

LOW-TEMPERATURE AUTOFRETTAGE: AN IMPROVED TECHNIQUE TO ENHANCE THE FATIGUE RESISTANCE OF THICK-WALLED TUBES AGAINST PULSATING INTERNAL PRESSURE

Abstract— It is shown that autofrettage at low temperatures is superior to autofrettage at room temperature in enhancing the fatigue resistance of thick-walled tubes against pulsating internal pressure. The physical reason is based on the well-known temperature dependence of the mechanical behaviour of metals and alloys which generally exhibit an enhancement of both the yield stress and strain hardening behaviour at lower temperatures. As a consequence, significantly larger compressive residual hoop stresses can be introduced during pressurization at low temperatures than at room temperature. Experimental data obtained on thick-walled tubes of the metastable austenitic stainless steel AISI 304 L which were subjected to pulsating internal pressure at room temperature after autofrettage at temperatures between-110°C and room temperature are presented. These data demonstrate convincingly the advantages offered by low-temperature autofrettage in enhancing both the fatigue life in the finite-life region and the fatigue endurance limit in comparison with autofrettage at room temperature. In conclusion, some specific materials requirements for optimum low-temperature autofrettage performance are discussed.     

Fabrication and properties of Lotus-type porous nickel-free stainless steel for biomedical applications

Lotus-type porous Fe–25 wt.%Cr, Fe–23 wt.%Cr–2 wt.%Mo alloys and type AISI 446 stainless steel were fabricated by continuous zone melting technique in pressurized hydrogen and helium gas. The porosity of the samples varied in the range 44–48% and the mean pore size values obtained (145–374 μm) are in the biomedical field desired range. The fabricated Lotus-type porous nickel-free stainless steel was nitrided at high temperature up to the nitrogen concentration of 1.0 wt.% and this amount resulted to be sufficient for maintaining almost single-phase austenitic structure at room temperature. The combination of very small magnetic susceptibility, light weight, mechanical properties close to the human cortical bone, together with the known good enough corrosion resistance of high nitrogen nickel-free stainless steel, makes this Lotus-type porous Fe–Cr–N alloys very attractive for bone implant applications.     

A study on the diffusion kinetics of borides on boronized Cr-based steels

In the present study, kinetics of borides formed on AISI H13 hot work tool and AISI 304 stainless steels have been investigated. Boronizing treatment was carried out in slurry salt bath consisting of borax, boric acid and ferrosilicon at temperature range of 1073–1223 K for 3, 5 and 7 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B, CrB and Ni₃B. Metallographic studies revealed that the boride layer has a flat and smooth morphology in the 304 steel while H13 steel was a ragged morphology. Depending on temperature and layer thickness, the activation energies of boron in 304 and H13 steels were found to be 253.35 and 244.37 kJ mol⁻¹, respectively.     

Measurement of the Thermal Conductivity of Stainless Steel AISI 304L Up to 550 K

New measurements of the thermal conductivity of stainless steel AISI 304L over the temperature range 300 to 550 K are reported. To perform the measurements, the transient hot-wire technique was employed, with a new wire sensor. The sensor makes use of a soft silicone paste material and of two thin polyimide films, between the hot wires of the apparatus and the stainless steel specimen. The transient temperature rise of the wire sensor is measured in response to an electrical heating step over a period of 40 s to 2 s, allowing an absolute determination of the thermal conductivity of the solid, as well as of the polyimide film and the silicone paste. The method is based on a full theoretical model with equations solved by a two-dimensional finite-element method applied to the exact geometry. At the 95% confidence level, the standard deviation of the thermal conductivity measurements is 0.6%, while the standard uncertainty of the technique is less than 1.5%.     

Study of ageing-induced α′-martensite formation in cold-worked AISI type 304 stainless steel using an acoustic emission technique

-martensite formation during cooling of cold-worked and aged AISI type 304 stainless steel has been studied by an acoustic emission technique. The ageing was carried out at 673 K for 1 h. A substantial amount of acoustic emission generated during cooling of cold-worked and aged AISI 304 stainless steel specimens compared to negligible acoustic emission observed during cooling (after ageing) of annealed AISI 304, annealed AISI 316 and cold-worked AISI 316 stainless steel specimens, was attributed to the -martensite formation from cold-worked 304 stainless steel specimens. The extent of martensite formation was relatively higher for 10% and 50% cold-worked specimens and lower for 20%–40% cold-worked specimens. The temperature range of martensite formation, as detected by the acoustic emission technique lies between 603 and 466 K. The formation of -martensite has been established to occur by a shear process.     

不锈钢冷加工形变诱发马氏体相变及其腐蚀行为

在实验室和现场对经过不同温度，不同方式，不同程度冷加工的奥氏体304不锈钢进行马氏体相变量的检测，研究了冷加工与马氏体相变的关系。通过浸泡试验和金相显微镜研究了304不锈钢在腐蚀过程中马氏体含量的变化。结果表明，冷加工能产生不同程度的马氏体相变；在腐蚀过程中，马氏体相存在优先溶解。     

Transient liquid-phase insert metal bonding of Al2O3 and AISI 304 stainless steel

Transition liquid-phase insert metal bonding of Al2O3 and AISI 304 stainless steel based materials is investigated. This joining technique allows the continuous replenishment of the active solute which is consumed by the chemical reaction that occurs at the ceramic/filler metal interface. Replenishment is facilitated by employing a sandwich of filler materials comprising tin-based filler metal and amorphous Cu50Ti50 or NiCrB interlayers. During Al2O3/AISI 304 stainless steel bonding, the highest shear strength properties are produced using a bonding temperature of 500 °C. Thick reaction layers containing defects form at the ceramic/filler material interface when higher bonding temperatures are applied. Bonding at temperatures above 500 °C also increases the tensile residual stress generated at the periphery of Al2O3/AISI 304 stainless steel joints. The shear strength of joints produced using NiCrB interlayers markedly increased following heat treatment at 200 °C for 1.5 h. Heat treatment had little influence on the shear strength of the joint produced using Cu50Ti50 interlayers. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal Conductivity of Reference Solid Materials

The thermal conductivity of three thermal-conductivity reference materials, Pyrex 7740, Pyroceram 9606, and stainless steel AISI 304L, has been studied. The technique employed is the transient hot-wire technique, and measurements cover a temperature range from room temperature up to 570K. The technique is applied here in a novel way that eliminates all remaining contact resistances. This allows the apparatus to operate in an absolute way. The method makes use of a soft silicone paste material between the hot wires of the technique and the solid of interest. Measurements of the transient temperature rise of the wires in response to an electrical heating step in the wires over a period of 20s up to 20s allow an absolute determination of the thermal conductivity of the solid, as well as of the silicone paste. The method is based on a full theoretical model with equations solved by a two-dimensional finite-element method applied to the exact geometry. At the 95% confidence level, the standard deviation of the thermal conductivity measurements is 0.1% for Pyrex 7740, 0.4% for Pyroceram 9606, and 0.2% for stainless steel AISI 304L, while the standard uncertainty of the technique is less than 1.5%.     

The specific heat of fifteen stainless steels in the temperature range 4K-30K

The specific heats of fifteen commercial and special grades of stainless steel, containing 12% to 25% Cr and 9% to 20% Ni, have been measured between 4 and 30 K. The results show that the low temperature specific heats are particularly sensitive to the relative amounts of Ni and Cr in the alloys; a high proportion of Ni to Cr generally resulting in a high specific heat. Near 4 K, however, alloys containing a high proportion of Cr show evidence of superparamagnetism with the result that their specific heats are anomalously high with values approaching those of alloys containing larger quantities of Ni. Values obtained for the electronic specific heat coefficients of these steels suggest a maximum in the density of states of fcc-iron at about 8.3 electrons per atom.     

Corrosion of a stainless steel handrail

AISI 304 and 304L stainless steels are “workhores” grades of austenitic stainless steel frequently used in architectural applications, as well as in cookware, appliances, and numerous other applications where resistance to corrosion is required. This paper examines a corrosion failure (the appearance of rustlike stains on the surface) of a 304 stainless steel handrail that appears to have occurred as a result of contamination during the fabrication process.     

Surface hardness improvement of plasma-sprayed AISI 316L stainless steel coating by low-temperature plasma carburizing

Low-temperature carburizing below 773 K of austenite stainless steel can produce expanded austenite, known as S-phase, where surface hardness is improved while corrosion resistance is retained. Plasma-sprayed austenitic AISI 316L stainless steel coatings were carburized at low temperatures to enhance wear resistance. Because the sprayed AISI 316L coatings include oxide layers synthesized in the air during the plasma spraying process, the oxide layers may restrict carbon diffusion. We found that the carbon content of the sprayed AISI 316L coatings by low-temperature carburizing was less than that of the AISI 316L steel plates; however, there was little difference in the thickness of the carburized layers. The Vickers hardness of the carburized AISI 316L spray coating was above 1000 HV and the amount of specific wear by dry sliding wear was improved by two orders of magnitude. We conclude that low-temperature plasma carburizing enabling the sprayed coatings to enhance the wear resistance to the level of carburized AISI 316L stainless steel plates. As for corrosion resistance in a 3.5 mass% NaCl solution, the carburized AISI 316L spray coating was slightly inferior to the as-sprayed AISI 316L coating.