Strength and toughness tradeoffs for an ultrafine-grain size ferrite/cementite steel produced by warm-rolling and annealing

For an ultrafine grain ferrite/cementite (UGF/C) steel, the Charpy impact energy was measured at temperatures from 373 K to 4.2 K, and tensile tests were carried out at temperatures between 323 K and 77 K. For the steel with annealed microstructure, the ductile-to-brittle transition appearance temperature (DBTT) was lower than the Charpy transition temperature (CTT). With increasing annealing time at 873 K, the DBTT and the CTT increased, and the DBTT approached the CTT. The DBTT decreased with decreasing effective grain size. The effective grain size correlated to the grain size of the larger grain size peak in the distribution of grains with {1 0 0} planes. The annealed microstructures had higher yield strength for equivalent toughness (including upper shelf energy, DBTT and CTT) compared to the conventional ferrite/pearlite steel.     

Effects of acicular ferrite on charpy impact properties in heat affected zones of oxide-containing API X80 linepipe steels

This study was concerned with effects of acicular ferrite on Charpy impact properties in heat affected zones (HAZs) of two API X80 linepipe steels containing oxides. In the one steel, Mg and O₂ were additionally added to form a larger amount of oxides than the other steel, which was a conventional X80 steel containing a considerable amount of Al and Ti. Various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs of 35 kJ cm⁻¹ and 60 kJ cm⁻¹. Oxides present in the API X80 linepipe steels were complex oxides whose average size was 1-2 μm, and the number of oxides increased with increasing amount of Mg and O₂. The volume fraction of acicular ferrite present in the steel HAZs increased with increasing number of oxides, and decreased with increasing heat input. When the volume fraction of acicular in the HAZ was higher than 20%, Charpy impact energy at −20 °C was higher than 100 J as the ductile fracture mode was dominant. Particularly in the steel HAZs having a larger amount of oxides, Charpy impact properties were excellent because oxides worked as nucleation sites of acicular ferrite during welding. Charpy impact properties of the HAZs could be well correlated with the volume fraction of acicular ferrite and number of oxides under different heat input conditions.     

高氮低镍奥氏体不锈钢的研究进展

高氮低镍奥氏体不锈钢是一种以氮代镍来获得稳定奥氏体组织的新钢种,它不但可以提高不锈钢的综合性能、节约镍资源,而且可以解决含镍较高的不锈钢用于人体时造成的镍过敏问题,在生物医学领域应用潜力巨大.综述了高氮低镍奥氏体不锈钢的发展历史和现状、不锈钢中氮的作用及高氮钢的主要制备工艺.     

Influence of sulphur additions on impact properties of bainitic or martensitic steels

Abstract

Effects of sulphur addition on the Charpy impact properties of various continuously cooled bainitic steels with different prior austenite grain size, hardness, and content of retained austenite were investigated and compared with martensitic steels. The impact properties of 1473 K austenitised bainitic steels were improved with increasing sulphur content up to 0·1 wt-%, while the impact properties of martensitic steels were deteriorated with increasing sulphur content. The crack initiation energy of bainitic steels increased with the increase of sulphur content because the structure units surrounded by the high angle boundaries were refined with the increase of manganese sulphide inclusions which caused the expansion of ductile fracture area. On the other hand, the impact energy, particularly the crack propagation energy, of martensitic steels decreased with increasing sulphur content because the nucleation sites of voids increased with the increase of manganese sulphide inclusions in the ductile fracture region.     

A comparative study of ductile-brittle transition behavior and fractography of P91 and P92 steel

The modified 9Cr-1Mo (P91) and 9Cr- 0.5Mo- 1.8W (P92) steel used in fast breeder reactor is exposed to irradiation during service which severely affects the dynamic fracture resistance by increasing the ductile to brittle transition temperature (DBTT). Thus, even at room temperature, the steel can become brittle and prone to cracking. In the present investigation, to elucidate the influence of low temperature on the DBTT, Charpy toughness test was performed on creep strength enhanced ferritic P91 and P92 steel. Lower DBTT was observed for the P92 steel as compared to P91 steel. To find the mode of fracture, the fractured Charpy toughness specimens were investigated using a field electron scanning electron microscope (FESEM). The fracture surface revealed the brittle mode of fracture at a lower temperature for both the steels while the mixed mode of fracture was noticed at room temperature and above.     

Nickel-free austenitic stainless steels for medical applications

Abstract

The adverse effects of nickel ions being released into the human body have prompted the development of high-nitrogen nickel-free austenitic stainless steels for medical applications. Nitrogen not only replaces nickel for austenitic structure stability but also much improves steel properties. Here we review the harmful effects associated with nickel in medical stainless steels, the advantages of nitrogen in stainless steels, and emphatically, the development of high-nitrogen nickel-free stainless steels for medical applications. By combining the benefits of stable austenitic structure, high strength and good plasticity, better corrosion and wear resistances, and superior biocompatibility compared to the currently used 316L stainless steel, the newly developed high-nitrogen nickel-free stainless steel is a reliable substitute for the conventional medical stainless steels.     

Effect of microstructural evolution in bainite,martensite, and δ ferrite on toughness of Cr–2W steels

Abstract

To investigate the effect of microstructural evolution during heat treatment in newly developed Cr–2W ferritic steels containing from 2 to 15%Cr, Charpy impact testing was carried out after quenching, tempering, and subsequent prolonged aging at 873 K for 3600 ks (1000 h). The ductile to brittle transition temperature (DBTT) and the upper shelf energy were determined as functions of heat treatment and Cr concentration. The fracture sequence was examined on fractographs obtained using scanning electron microscopy. The steels consisted of bainite, martensite, and δ ferrite phases and carbide precipitates, depending on the Cr concentration. The DBTT decreased considerably after tempering compared with after quenching, but increased again somewhat after subsequent prolonged aging. The upper shelf energy exhibited converse behaviour. Concerning the effect of alloy phases, the DBTT was increased significantly by the formation of δ ferrite and was slightly higher for the martensite than for the bainite. The effects of the alloy phases and heat treatments on the DBTT are discussed by considering the microstructural evolution, such as dislocation recovery and carbide precipitation, during heat treatment.

MST/1573     

Tensile behaviour of duplex stainless steel at low temperatures

Abstract

The tensile behaviour of the ferrite and austenite phases of Fe–22Cr–5Ni (wt-%) duplex stainless steel containing a maximum of 17·2% austenite was investigated in the temperature range 65–298 K. The results indicate that mechanical twinning occurred in the testing temperature range, and that austenite impeded the growth of twinning. Mechanical twinning in ferrite was well decorated with a ‘dislocation shell’, and the density of dislocations at the coherent twin boundary and within a twin was much higher than in the matrix above the ductile–brittle transition temperature (DBTT). This supported the occurrence of slip localisation next to coherent twin boundaries. Dislocations in the material with no austenite tested below the DBTT were characterised by coplanar slip dislocation on the { 110} plane, and both coplanar slip on { 110} and cross-slip dislocations were observed above the DBTT. Dislocation in ferrite was negligibly affected by the presence of austenitic particles. Strain induced martensite transformation occurred in austenitic particles at or below 220 K, and the characteristics of the transformation were essentially similar to those in type 304 stainless steel. The DBTT of the material was lowered from ～140 to 110 K in the presence of austenite, independent of the volume fraction of austenite. This suggests that the decrease in the DBTT of the material was mainly due to austenite scavenging carbon and other interstitial elements from the ferritic matrix. The fracture of the material at low temperatures was primarily controlled by the fracture of twin boundaries in ferrite.     

Evaluation on toughness degradation of Cr–Mo–V steel using miniaturized impact specimen technology

Miniaturized specimen technology is inevitable when the amount of available material for test is limited. In this study, miniaturized Charpy V-notched specimens of 1Cr–1Mo–0.25 V rotor steel with five different aging periods were artificially prepared by an isothermal aging heat treatment at 630°C and tested. For the miniaturized specimens, two different types of specimens with or without side groove were utilized. A correlation between the ductile brittle transition temperature (DBTT) obtained by the miniaturized specimen and that by the standard specimen was investigated. In addition, the relationship between fracture toughness and DBTT by the miniaturized specimen of degraded 1Cr–1Mo–0.25 V rotor steels was proposed.     

Plastic deformation and fracture behaviors of nitrogen-alloyed austenitic stainless steels

The plastic deformation and fracture behaviors of two nitrogen-alloyed austenitic stainless steels, 316LN and a high nitrogen steel (Fe–Cr–Mn–0.66% N), were investigated by tensile test and Charpy impact test in a temperature range from 77 to 293 K. The Fe–Cr–Mn–N steel showed ductile-to-brittle transition (DBT) behavior, but not for the 316LN steel. X-ray diffraction (XRD) confirmed that the strain-induced martensite occurred in the 316LN steel, but no such transformation in the Fe–Cr–Mn–N steel. Tensile tests showed that the temperature dependences of the yield strength for the two steels were almost the same. The ultimate tensile strength of the Fe–Cr–Mn–N steel displayed less significant temperature dependence than that of the 316LN steel. The strain-hardening exponent increased for the 316LN steel, but decreased for the Fe–Cr–Mn–N steel, with decreasing temperature. Based on the experimental results and the analyses, a modified scheme was proposed to explain the fracture behaviors of austenitic stainless steels.     

Nondestructive Evaluation of Ductile to Brittle Transition Temperature for Martensitic Steels Based on Magnetic Measurements

Interest in magnetic nondestructive evaluation (NDE) methods for nuclear reactors materials to quantify material embrittlement and ensure reactor safety is increased. In this paper, ductile–brittle transition temperature (DBTT) and temperature dependence of the coercive field (\(H_{C})\) and magnetization (M) of martensitic steels (F82H, Eurofer97, SCRAM, T91 and cold rolled T91 steels) were investigated from 100 to 350 K to non-destructively evaluate their DBTT. Results show that a characteristic temperature in both ln(\(H_{C})\) and d(lnM)/d(1/T) vs temperature curves coincides well with the corresponding DBTT measured by Charpy impact test. The curves decrease linearly above and below DBTT respectively, the absolute slope above DBTT is larger than that below. The phenomenon is closely related to the different motion status and the pinning strength of dislocations and magnetic domain wall in the ductile and brittle regions. Results indicate the magnetic evaluation method could be a promising technique with good reproducibility and high sensitivity to evaluate DBTT of martensitic steels used in nuclear reactors.     

Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (T_P1) of 1400 °C, then reheating to different second peak temperatures (T_P2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.     

23Cr-14Ni高氮奥氏体不锈钢σ相析出行为EI北大核心CSCD

采用经验公式、热力学计算方法、Gleeble热/力模拟实验技术,结合光学显微镜、扫描电镜及透射电镜分析,研究了23Cr-14Ni高氮奥氏体不锈钢中σ相的析出行为。结果表明,23Cr-14Ni高氮奥氏体不锈钢中σ相可在960~1030℃析出,高于1050℃溶解。σ相析出具有异常快速的动力学特征,在经过1030℃保温1 min固溶处理后,σ相可直接从奥氏体晶界快速析出,析出先于碳氮化物相。σ相析出动力学行为及相对碳氮化物的析出次序和传统奥氏体不锈钢显著不同。铬、锰、钼元素含量较高且钼元素在晶界处偏聚提高了σ相平衡析出温度,是加速σ相析出的主要原因。     

Low-temperature mechanical and magnetic properties of the reduced activation martensitic steel

Mechanical and magnetic properties as well as their relationship in the reduced activation martensitic (RAM) steel were investigated in the temperature range from --90°C to 20°C. Charpy impact tests show that the ductile-to-brittle transition temperature (DBTT) of the RAM steel is about --60°C. Low-temperature tensile tests show that the yield strength, ultimate tensile strength and total elongation values increase as temperature decreases, indicating that the strength and plasticity below the DBTT are higher than those above the DBTT. The coercive field (H_C) in the scale of logarithm decreases linearly with the increasing temperature and the absolute value of the slope of lnH_C versus temperature above the DBTT is obviously larger than that below the DBTT, also confirmed in the T91 steel. The results indicate that the non-destructive magnetic measurement is a promising candidate method for the DBTT detection of ferromagnetic steels.     

Influence of grain boundary carbide thickness and grain size on cleavage fracture strength and Charpy impact behaviour of steels

Abstract

The cleavage fracture strengths and Charpy impact transition temperatures of plain C–Mn steels (0·12%C and 1–1·4%Mn) having ferrite–pearlite microstructures have been determined for ranges of grain size and grain boundary carbide thickness. Using appropriate heat treatments, ferrite grain size and carbide thickness were varied independently. Refining the ferrite grain size or grain boundary carbide thickness increased the fracture strength and decreased the impact transition temperature. Of the current theories for brittle fracture, an equation recently derived by Petch was found to give the most satisfactory agreement with the experimental data for cleavage strength.

MST/1424     

Effect of silicon and aluminium in ferrite on tensile and impact properties

Two ferritic interstitial-free steels with approximately the same amount of solid solution strengthening by addition of 2?wt-% silicon and 4?wt-% aluminium are investigated using quasi-static tensile and dynamic impact tests. The addition of 2?wt-% silicon (2Si) results in brittle fracture in V-notched Charpy impact tests at ambient temperature, whilst the 4?wt-% aluminium-containing (4Al) steel has high absorbed energy of 320?±?12?J?cm^?2. In addition, the 4Al steel has a ductile-to-brittle-transition temperature (DBTT) ～60°C lower than the 2Si steel. It is proposed that the addition of silicon suppresses dislocation cross-slip at high strain rate and is responsible for the observed cleavage fracture and high DBTT in the 2Si steel. The ease of dislocation slip in the 4Al steel increases the impact toughness.     

Mechanical properties of as-cast microalloyed steels produced via investment casting

Tensile and room temperature Charpy V-notch impact tests were used to evaluate the variations in the as-cast mechanical properties of a low-carbon steel produced via shell mould investment casting and containing combinations of vanadium, niobium and titanium. Tensile results indicate that the yield strength and ultimate tensile strength (UTS) have increased up to respectively 615 MPa and 770 MPa due to the fine-scale microalloy precipitates in the microalloyed samples. Room temperature impact test results show that while addition of vanadium individually has not changed the impact energy, Nb has decreased it considerably. However, examination of fracture surfaces reveals that all microalloyed samples have failed by transgranular cleavage. Based on the transmission electron microscope (TEM) studies, it seems that carbonitrides being greater than 50 nm in size and formed along prior austenite grain boundaries before γ transformation are responsible for the observed reduction in impact energies and brittle fracture. In comparison to sand mould casting, the yield and UTS obtained from investment casting are superior. Furthermore, although the impact energies of Nb-containing alloys are approximately the same as those obtained from sand moulds, the impact energy of the alloy containing only vanadium has improved considerably.     

Corrosion-resistant analogue of Hadfield steel

The concept of alloying austenitic steels with carbon + nitrogen is used for the development of a corrosion-resistant austenitic CrMn steel having an impact wear resistance close to that of the Hadfield steel. A higher stabilization of the austenitic phase by C + N, as compared to carbon or nitrogen alone, is substantiated by ab initio calculation of the electron structure, measurements of the concentration of free electrons and calculations of the phase equilibrium. Based on these results, the compositions (mass%) Cr18Mn18C0.34N0.61 and Cr18Mn18C0.49N0.58 were melted and tested along with Hadfield steel Mn12C1.2. Mechanical tests have shown that, as compared to the Hadfield steel, the experimental steels possess a higher strength, plasticity, hardness and the same resistance to impact wear. TEM studies of the surface layer after impact treatment revealed a mixture of the amorphous phase, nanocrystals and fine-twinned austenite. At the same time, using Mössbauer spectroscopy of conversion electrons, the ferromagnetic ordering was found in the surface layer of up to 10 μm in depth, which is the sign of the strain-induced martensitic phase. The hypothesis of a transition from the low-spin to the high-spin state of the iron atoms within the thin twins in austenite was proposed in order to interpret the discrepancy between TEM and Mössbauer studies. Potentiodynamic measurements and immersion tests show that the CrMnCN steels possess a significantly higher pitting potential and resistance to general corrosion in comparison with Hadfield steel.     

Hypervelocity impact into a high strength and ductile steel alloy

The unusual properties of nitrogen alloyed austenitic steels have been reported in the recent two decades in many papers. In this work it is aimed to investigate a P900 alloy subjected to a hypervelocity impact. The P900 alloy was work hardened to a medium hardness of 380 HV30 by cold expansion of a ring. Tungsten heavy metal and pure tungsten were used as projectile materials. The geometry of the long rods was 3 mm × 30 mm for diameter and length, respectively. Ballistic tests were performed with a two-stage light-gas gun at velocities from about 2000 m/s up to about 4500 m/s. It was found that two kinds of crater geometry are possible depending on the tendency of the projectile material to adiabatic shear banding or brittle fracture. The brittle W material achieved a deeper crater than the shear band forming W heavy alloy.     

Effects of yttrium on microstructure and properties of reduced activation ferritic-martensitic steel

This study investigates the inclusions, microstructure, tensile properties, and impact toughness of reduced activation ferritic/martensitic (RAFM) steels with different Y contents (0.006, 0.034 and 0.071?wt-%). Owing to the pinning of the grain boundary to the Y inclusions (which refines the original austenite grain size) and an existing Fe–Cr–Ta–Y–S–O phase, the tensile strength at both room and high temperatures increased with increasing Y content (below 0.071%). Increasing the Y content to 0.034 wt-% decreased the ductile-to-brittle transition temperature (DBTT). However, when the Y content reached 0.071?wt-%, the DBTT increased as the Y precipitated into blocky yttrium-rich inclusions. The microstructure, tensile properties, and impact toughness of the RAFM steel were optimised at a Y content of approximately 0.034?wt-%.