0.3C–CrMoV(ESR) Steel: A New Ultrahigh Strength Steel

0.3C–CrMoV(ESR) steel is a new ultrahigh strength medium-carbon low alloy steel which is developed as a cost effective material for space applications. It is a quenched and tempered steel and upon heat treatment it develops an ultimate tensile strength (UTS) of up to 1700 MPa with a corresponding 0.2% proof strength of 1400 MPa and a tensile elongation of 8%. The steel is processed through the secondary melting practice of electro-slag refining (ESR). Inoculation with niobium is employed to reduce the grain size of the steel. ESR-cast ingots are subjected to thermo-mechanical processing like hot forging, rolling and ring-rolling to realize product forms such as plates, rings and bars. It has excellent cold and hot working characteristics, and it is amenable to welding processes such as gas tungsten arc welding and plasma arc welding. Filler wire having the same chemical composition as that of the base metal is used, and a welding efficiency close to 100% is achievable. In this paper, brief outline on the steel’s characteristics such as chemical composition, processing details, mechanical properties of base metal and weld metal, etc. is provided.     

Melting and Thermomechanical Processing of High Strength 0.3%C–CrMoV (ESR) Steel

0.3%C–CrMoV steel were processed through electric arc furnace melting followed by electro slag refining. 2800 mm diameter class of rolled rings and 8 mm thick plates required for fabrication of solid rocket booster motorcase were realized. Tensile and fracture toughness properties were evaluated as a part of characterization of the steel. Low fracture toughness in initial melts was investigated using optical and scanning electron microscopy. Modifications in methods of alloying additions/processing were suggested and incorporated to achieve the desired mechanical properties in industrial scale melts. Process was also fine-tuned by incorporating additional thermomechanical working and heat treatment cycles to achieve the required mechanical properties. Hardening cycle of 925 °C for 1 h followed by oil quenching and tempering cycle of 505 °C for 2 h followed by oil quenching was found to result in optimum combination of mechanical properties. Repeatability in processing and consistency in achieving the mechanical properties of the steel at industrial scale was demonstrated by processing 24 melts of 6 tons each into large size rings and plates.     

Effect of Initial Microstructure on Impact Toughness of 1200 MPa-Class High Strength Steel with Ultrafine Elongated Grain Structure

A medium-carbon low-alloy steel was prepared with initial structures of either martensite or bainite. For both initial structures, warm caliber-rolling was conducted at 773 K (500 °C) to obtain ultrafine elongated grain (UFEG) structures with strong 〈110〉//rolling direction (RD) fiber deformation textures. The UFEG structures consisted of spheroidal cementite particles distributed uniformly in a ferrite matrix of a transverse grain size of about 331 and 311 nm in samples with initial martensite and bainite structures, respectively. For both initial structures, the UFEG materials had similar tensile properties, upper shelf energy (145 J), and ductile-to-brittle transition temperatures 98 K (500 °C). Obtaining the martensitic structure requires more rapid cooling than is needed to obtain the bainitic structure and this more rapid cooling promote cracking. As the UFEG structures obtained from initial martensitic and bainitic structures have almost identical properties, but obtaining the bainitic structure does not require a rapid cooling which promotes cracking suggests the use of a bainitic structure in obtaining UFEG structures should be examined further.     

Development of TiMicroalloyed 600 MPa Hot Rolled High Strength Steel

 A high strength steel with tensile strength on the order of 710MPa had been development successfully with only addition of titanium alloy element based on a low carbon steel. The results show the hot deformation accelerates ferrite and pearlite transformation and retards bainite transformation under continuous cooling condition. The microstructure of this steel is mainly composed of fine-grained ferrite and carbides distributed along the ferrite grain boundaries. The yield and tensile strengths of steels are about 620～650MPa and 720～740MPa, respectively, and the values of strain hardening exponent (n) and plastic strain ratio (r) are 0.12 and 0.80, respectively, thus providing well-matched strength with toughness. In short, the fine-grained ferrite and TiC nano-precipitates play an effective role in strengthening the steel.     

Design of TRIP Steel With High Welding and Galvanizing Performance in Light of Thermodynamics and Kinetics

 A new type of transformation induced plasticity (TRIP) steel with not only high strength and high ductility but also superior welding and galvanizing properties was designed and developed recently. Low carbon and low silicon content were preliminarily selected with the aim of meeting the requirements of superior quality in both welding and galvanizing. Phosphorus was chosen as one of the alloying elements, because it could reduce carbon activity in cementite and increase the stability of austenite. In addition, the possibility of phosphorus segregating at grain boundary was also discussed by thermodynamics as well as kinetics. Phase diagram was estimated at high temperature and the composition of the steel was then selected in the hyperperitectic range to avoid problems, which might occur in sheet steel continuous casting. Phase diagram in the inter critical temperature was estimated for the steel to obtain the starting temperature of fast cooling. For understanding the minimum rate of fast cooling, pearlite growth kinetics was calculated with self developed diffusion coefficients of elements in grain boundary. Overaging temperature was determined through the calculation of T0 temperature by both equilibrium and para equilibrium assumptions, which was different from the current determination, which is only based on an equilibrium estimation.     

Effect of Boron on Delayed Fracture Resistance of MediumCarbon High Strength Spring Steel

The delayed fracture behavior of mediumcarbon high strength spring steel containing different amounts of boron (0000 5%, 0001 6%) was studied using sustained load delayed fracture test. The results show that delayed fracture resistance of boron containing steels is higher than that of conventional steel 60Si2MnA at the same strength level and it increases with the increase of boron content from 0000 5% to 0001 6%. The delayed fracture mode is mainly intergranular in the boron containing steels tempered at 350 ℃, which indicates that the addition of boron does not change the fracture character. However, the increase of boron content enlarges the size of the crack initiation area. Further study of phase analysis indicates that most boron is in solid solution, and only a very small quantity of boron is in the M3(C, B) phase.     

Studies on Welding and Sensitization of Chrome–Manganese Austenitic Stainless Steel

The standard composition of austenitic stainless steel (ASS) includes Cr and Ni. However, due to rising cost of Ni, search for suitable alternative is always on. Manganese, on account of being an austenite stabilizer, can be considered as a replacement of Ni. Chrome–manganese steel (part of 200 series of ASS) contains chromium (≤15%) and low nickel. However, there is not enough confidence about their service life. Present work is an attempt to study the effect of welding and sensitization characteristics of this steel. The heat affected zone (HAZ) is identified microstructurally as per ASTM standard A262 Practice A test. Time temperature sensitization (TTS) diagram is established and critical cooling rate (CCR) is estimated. double loop electrochemical potentiokinetic reactivation (DLEPR) technique is used to quantify degree of sensitization (DOS) for heat treated and welded samples. The DOS for thermally aged sample at 750°C for 180 min is found to be 32.56% whereas for welded sample it is 31.30%. The results are discussed and mechanism of micro-structural changes due to welding of such steel is suggested.     

Effect of Coiling Temperature on the Microstructure and Mechanical Properties of Hot-Rolled Ti–Nb Microalloyed Ultra High Strength Steel

A novel low carbon Ti–Nb microalloyed hot rolled steel with minimum yield strength of 700 MPa and good balance of stretch-flangeability and impact toughness has been developed by controlled thermo-mechanical processing following thin slab direct rolling route. In the present work, the effects of two coiling temperatures on the resulting microstructure, micro-texture and mechanical properties on this Ti–Nb microalloyed steel have been studied. It is observed that increase in coiling temperature from 520 to 580 °C significantly affects the mechanical properties. Higher dislocation density and increased precipitation along with slightly smaller grain size is observed for 580 °C coiling temperature resulting in about 50 MPa increase in yield and tensile strengths as compared to 520 °C coiling temperature.     

Interaction of Hydrogen and Retained Austenite in Bainite／Martensite Dual-Phase High Strength Steel

SymbolList　　DA———Apparentdiffusioncoefficient;　　DL———Latticediffusioncoefficient;　　EaD———Diffusionactivationenergyofhydrogeninnormallattice ;　　EaT———Trapactivationenergy ;　　EB———Trapbindingenergy ;　　ES———Saddlepointenergy ;     

Two Parameter Weibull Analysis of the Effect of Chemical Modification of Al–7si–0.3 Mg Alloy on Ultimate Tensile Strength

Double oxide films are one of the primary reasons that cause casting defects particularly in dross forming alloys. It is not clear whether the beneficial effect of modification melt treatment in Al–Si alloys is entirely due to the transformation of acicular eutectic Si to fibrous morphology. In the present work, the effect of chemical modification of Al–7Si–0.3 Mg alloy on casting reliability was assessed from the Weibull analysis of tensile strengths. The findings show that the modification melt treatment of Al–Si alloy has a beneficial effect as indicated by consistently higher Weibull modulus. Apart from transformation in the silicon morphology, modifiers decrease the scatter present as a result of various defects, particularly the bi-film, resulting in higher reliability for the modified castings.     

Assessment of Arc Welding Process Through the Combination of TOPSIS–AHP Methods with Fuzzy Logic

In this work we have shown that how combining the multi-criteria decision making (MCDM) process with fuzzy logic can be very efficient to solve decision problems in which there are criteria of different natures. This combination is applied here to solve a real decision problem of great interest in structural steel welding which is used widely in industry. The problem consists of selecting the best electric arc welding processes for a plate thickness greater than 12 mm. Because facing a decision problem of this type involves, on one hand, quantitative criteria (thermal efficiency, productivity, minimum power, etc.) and on the other hand, qualitative criteria (fatigue of welder, process learning, reliability, etc.), not only will it be necessary to apply a MCDM process, but it should also be combined with fuzzy logic with the aim of being able to perform the extraction of knowledge through a survey focused on experts. The Analytic Hierarchy Process has been used in order to obtain the weight of the criteria and, through the Technique for Order Preference by Similarity to Ideal Solution. The alternatives are also evaluated. There are no previous studies related to welding processes using the MCDM process with fuzzy logic.     

Resistance Spot Welding of Quenching and Partitioning (Q&P) Third-Generation Advanced High-Strength Steel: Process–Microstructure–Performance

Metallurgical and Materials Transactions A - This paper investigates process–microstructure–performance relationships in Q&P980 third-generation advanced high-strength steel...     

Microstructure and High Temperature Impression Creep Properties of Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) Alloys

The current study has investigated the influence of zirconium (Zr) addition to Mg–3Ca–xZr (x = 0.3, 0.6, 0.9 wt%) alloys prepared using argon arc melting on the microstructure and impression properties at 448–498 K under constant stress of 380 MPa. Microstructural analysis of as-cast Mg–3Ca–xZr alloys showed grain refinement with Zr addition. The observed grain refinement was attributed to the growth restriction effect of Zr in hypoperitectic Mg–3Ca–0.3 wt% Zr alloys. Heterogeneous nucleation of α-Mg in properitectic Zr during solidification resulted in grain refinement of hyperperitectic Mg–3Ca–0.6 wt% Zr and Mg–3Ca–0.9 wt% Zr alloys. The hardness of Mg–3Ca–xZr alloys increased as the amount of Zr increased due to grain refinement and solid solution strengthening of α-Mg by Zr. Creep resistance of Mg–3Ca–xZr alloys increased with the addition of Zr due to solid solution strengthening of α-Mg by Zr. The calculated activation energy (Q_a) for Mg–3Ca samples (131.49 kJ/mol) was the highest among all alloy compositions. The Q_a values for 0.3, 0.6 and 0.9 wt% Zr containing Mg–3Ca alloys were 107.22, 118.18 and 115.24 kJ/mol, respectively.     

Chemical Interactions of Alumina–Carbon Refractories with Molten Steel at 1823 K (1550 °C): Implications for Refractory Degradation and Steel Quality

A sessile-drop study was carried out on Al₂O₃-10 pct C refractory substrates in contact with molten iron to investigate possible chemical reactions in the system and to determine the influence of carbon and the role, if any, played by the presence of molten iron that can act both as a reducing agent and as a metallic solvent. These investigations were carried out at 1823 K (1550 °C) in argon atmosphere for times ranging between 15 minutes and 3 hours. We report the occurrence of chemical reactions in the Al₂O₃-10 pct C/Fe system, associated generation of CO gas, and carbon pickup by molten iron. Video images of the iron droplet started to show minor deviations after 30 minutes of contact followed by intense activity in the form of fine aluminum oxide whiskers emanating from the droplet and on the refractory substrate. The interfacial region also changed significantly over time, and the formation of small quantities of iron aluminide intermetallics was recorded after 30 minutes as a reaction product in the interfacial region. These chemical reactions also caused extensive penetration of molten iron into the refractory substrate. This study has shown that alumina cannot be treated as chemically inert at steelmaking temperatures when both carbon and molten iron are present simultaneously. These findings point to an additional reaction pathway during steelmaking that could have significant implications for refractory degradation and contamination of steel with reaction products.     

Glass-Forming Ability of Mg–Cu–Ni–Gd Bulk Metallic Glasses with High Strength

The glass-forming ability (GFA) of Mg–Cu–Ni–Gd alloy system was evaluated using copper mold casting. A three-dimensional composition map of Mg–Cu–Ni–Gd system with GFA over 6 mm was revealed, confirming that the Ni addition decreased the GFA of Mg–Cu–Gd system. The maximum Ni tolerance was about 6 at.% for the Mg–Cu–Ni–Gd BMGs with GFA over 6 mm. The compressive tests displayed that the Ni addition as small as 3.45 at.% could result in higher strength for the Mg–Cu–Gd BMGs. The Mg–Cu–Ni–Gd system with small Ni content can be balanced candidates for the Mg-based BMGs with both acceptable GFA and high strength.     

Industrial processing,microstructures and mechanical properties of Fe–(2–4)Mn (–0.85Mo)–(0.3–0.7)C sintered steels

Abstract

The potential of PM Mn steels has been established in laboratory experiments. This paper deals with sintering of Fe–(2–4)Mn–(0.3/0.7)C, also with 0˙85%Mo addition, in an industrial pusher furnace at 1180°C in an atmosphere of 25% hydrogen plus 75% nitrogen, obtained from a cryogenic liquid, giving an inlet dew-point of ?55 °C. Tensile, bend (including fatigue) and miniature Charpy specimens were sintered in flowing gases and in semiclosed containers with a getter of ferromanganese, carbon and alumina. The quenched and tem- pered state was investigated, as was sinter hardening (cooling rate of 55 K min ?1), simulated for comparison with slow cooling at 10 K min ?1. As there was no forma tion of oxide networks at the combination of sintering temperature and dewpoint, in accordance with the Ellingham–Richardson diagram for Mn oxidation/reduction, the use of semiclosed containers was superfluous. The quenched and tempered specimens were brittle. Sinter hardening lead to an improvement in mechanical properties. The reproducibility of tensile and TRS data was high for the sintered materials, characterised by Weibull moduli m of 12–41. All the alloy microstructures were complex and heterogeneous, consisting of, depending on the local manganese and carbon contents, the diffusive and non-diffusive transformation products (pearlite, bainite, martensite) and additionally ferrite and retained austenite. The highest mechanical properties in the entire range of compositions investigated in the furnace cooled state: yield, tensile and bend strengths of 499, 637 and 1280 MPa, respectively, with impact energy of 18 J, and tensile and bend strains of 1˙17 and 1.57%, were achieved for the Fe–2Mn–0.85Mo–0.5C alloy, marginally superior to Fe–2Mn–0.7C. For the sinter hardened Fe–4Mn–0.3C alloy yield, tensile and bend strengths were 570, 664 and 1263 MPa, respectively, at an acceptable impact energy of 14 J, with tensile and bend strains of 0.52% and 1.8%. Many of the results compare favourably with the requirements of MPIF standard 35. Mn is a more effective strengthening agent than either Ni or Cu, or their combination, though generally at reduced plasticity.     

Tailoring the Processing Route to Optimize the Strength–Toughness Combination of Pearlitic Steel

Metallurgical and Materials Transactions A - The present study fine tunes the processing route of a eutectoid steel to shape an optimum strength–toughness combination through appropriate...