Cr Mo Low Alloy Steel Research Articles

Influence of the Hot-Top Thermal Regime on the Severity and Extent of Macrosegregation in Large-Size Steel Ingots

The influence of hot-top designs with different heat capacities on the distribution of positive and negative macrosegregation was investigated on a 12 metric tonne (MT) cast ingot made using Cr-Mo low-alloy steel. The three-dimensional finite element modeling code THERCAST® was used to simulate the thermo-mechanical phenomena associated with the solidification process, running from filling the mold until complete solidification. The model was validated on an industrial-scale ingot and then utilized to evaluate the influence of the thermal history of the hot-top, a crucial component in the cast ingot setup. This assessment aimed to comprehend changes in solidification time, temperature, and heat flux—all of which contribute to the determination of macrosegregation severity. The results showed that preheating the hot-top had a minor effect on solidification time, while modifications of thermal conductivity in the hot-top region increased the solidification time by 31%, thereby significantly affecting the macrosegregation patterns. The results are discussed and interpreted in terms of the fundamental mechanisms governing the kinetics of solidification and macrosegregation phenomena.

Microstructure and properties of low alloy CrMo steel processed with varied isothermal temperatures

The microstructure and properties of low alloy CrMo steel after isothermal treatment at different temperatures were studied. The experiment process is to rapidly cool the samples that have been held at 940 ℃ for 2 h to a salt bath furnace at different temperatures (300–500 ℃) for isothermal treatment, and then air cooling to ambient temperature. The microstructure after isothermal treatment is granular bainite, which is a mixed microstructure composed of bainitic ferrite (BF) and martensite/austenite (M/A) island. The morphology, size and volume fraction of M/A islands change with the change of isothermal temperature. The mechanical properties are also closely related to isothermal temperature. With the increase of isothermal temperature, the impact toughness and yield strength (YS) decreased. By studying the effects of different strengthening mechanisms and multiphase on YS, the results show that YS mainly depends on BF. According to the SEM and EBSD characterization of the fracture, it is found that the stress in the sample is mainly concentrated near the M/A, which makes the crack easy to initiate on the M/A island or its interface. The brittle fracture tendency of materials is related to M/A island. The larger the size of the M/A island, the more likely brittle fracture will occur and the less ductile it will be.

Quantitative dependence of ductile-to-brittle transition on phosphorus grain boundary segregation for a novel Ni–Cr–Mo RPV steel

SA508-4 N steel is considered a novel Ni–Cr–Mo low-alloy steel for reactor pressure vessels. The low-temperature embrittlement in the steel is an important issue that needs to be addressed. Herein, the main aim of this work is to investigate the non-hardening embrittlement caused by grain boundary segregation (GBS) of P in a P-doped SA508-4 N steel with the use of impact testing in conjunction with Auger electron spectroscopy (AES) and microstructural characterization. The steel samples are quenched from 1000 °C, followed by tempering at 650 °C and subsequent ageing at different lower temperatures. The ductile-to-brittle transition temperatures (DBTTs) of the aged specimens are determined by mini-Charpy impact testing (specimen: 2.5 mm × 2.5 mm in cross section), and the GBS of P is evaluated using AES. The results show that there is a linear dependence of DBTT (oC) on the boundary concentration of P (Cp/at.%), when the steel hardness is fixed after the 650 °C-tempering (∼260 HV10), being expressed as DBTT=13.31Cp−339.28. Employing this relation, one can forecast the DBTT once the boundary level of P is known. With the DBTT−segregation relation in combination with segregation thermodynamics and kinetics, a time–temperature embrittlement diagram is also developed. This may be employed to predict the tendency of embrittlement in the steel, caused by GBS of P after serving at a temperature. It is believed that this study is helpful in monitoring the safe operation of devices manufactured with this novel low-alloy steel.

Effect of Underload Cycles on Oxide-Induced Crack Closure Development in Cr-Mo Low-Alloy Steel

Underload cycles with small load amplitudes below the fatigue crack growth threshold are dominantly considered as insignificant cycles without any influence on fatigue lifespan of engineering structural components. However, this paper shows that in some cases these underload cycles can retard the consequent crack propagation quite significantly. This phenomenon is a consequence of oxide-induced crack closure development during cyclic loading below the threshold. The experimentally described effect of fatigue crack growth retardation was supported by measurement of the width and the thickness of the oxide debris layer using the EDS technique and localized FIB cuts, respectively. Both the retardation effect and the amount of oxide debris were larger for higher number and larger amplitudes of the applied underload cycles. Crack closure measurement revealed a gradual increase of the closure level during underload cycling. Specimens tested in low air humidity, as well as specimens left with the crack open for the same time as that needed for application of the underload cycles, revealed no retardation effect. The results can improve our understanding of environmental effects on fatigue crack propagation and understanding the differences between the results of laboratory testing and the fatigue lives of components in service.

Study on impact fracture behavior of G18CrMo2-6 steel after tempering

The evolution of microstructure of low alloy CrMo steel has a great influence on the mechanical properties. Three different tempering heat treatment conditions were made on the material, and the influence of the microstructure on the impact toughness under different test temperatures was studied. The type, morphology, size and distribution of the second phase will evolve with the change of tempering temperature and time. Impact energy of all samples increases gradually from the lower shelf to upper shelf with the testing temperature increasing. In the lower shelf, the fracture is controlled by the crack nucleation. In the ductile-to-brittle transition temperature (DBTT) range, the fracture is determined by the size of the initial crack initiating on the second phase. In the upper shelf, the cleavage fracture is controlled by the propagation of the carbide crack into continuous grains, and the critical event for fracture is the plasticity of the matrix. The scattering of impact energy in DBTT range is caused by the uneven distribution and size of the brittle second phase.

Distinct beneficial effect of Sn on the corrosion resistance of Cr–Mo low alloy steel

In this work, the beneficial effect of Sn addition on the corrosion resistance mechanism of Cr–Mo low alloy steel was studied. Results demonstrated that Sn improves the corrosion resistance of the steel matrix mainly by influencing the microstructural transformation. Sn addition and the synergistic effect of Sn, Cr, and Mo promote the formation of α-FeOOH, SnO2, SnO, Cr(OH)3, and molybdates, lead to the improved protection and stability of the rust layer. This synergistic effect also endows the inner rust layer with cation selectivity, preventing the further penetration of Cl− and inhibiting the localized corrosion of steel.

FEM-thermodynamic simulation methodology to predict the influence of t8/5 on the coarse grain heat-affected zone of a Cr-Mo low-alloy steel pipe

Abstract This work aimed to develop a microstructure-predicting methodology (MPM) using coupled physical-FEM-thermodynamic simulation to predict the influence of cooling time from 800 to 500 °C (t8/5 – 15, 30, 80, and 210 s) on the microstructure and mechanical behavior of the coarse grain heat-affected zone (CGHAZ) of a Cr-Mo low alloy steel welded joint. The MPM was experimentally validated. The MPM certainly predicted that the increase of t8/5 caused a remarkable change in CGHAZ microstructure (martensite and bainite to ferrite with aligned and non-aligned martensite-austenite-carbide M-A-C) and also decreases the Vickers microhardness. The MPM and experimental data show that the use of welding parameters that increases t8/5 (reducing the cooling rate) can jeopardize the welded joint behavior. Besides, the MPM proved to be a suitable tool for estimating the microstructure and microhardness of the CGHAZ.

Hot Deformation Behaviour of Mn–Cr–Mo Low-Alloy Steel in Various Phase Regions

The deformation behaviour of a coarse-grained as-cast medium-carbon steel, alloyed with 1.2% Mn, 0.8% Cr and 0.2% Mo, was studied by uniaxial compression tests for the strain rates of 0.02 s−1–20 s−1 in the unusually wide range of temperatures (650–1280 °C), i.e., in various phase regions including the region with predominant bainite content (up to the temperature of 757 °C). At temperatures above 820 °C, the structure was fully austenitic. The hot deformation activation energies of 648 kJ·mol−1 and 364 kJ·mol−1 have been calculated for the temperatures ≤770 °C and ≥770 °C, respectively. This corresponds to the significant increase of flow stress in the low-temperature bainitic region. Unique information on the hot deformation behaviour of bainite was obtained. The shape of the stress-strain curves was influenced by the dynamic recrystallization of ferrite or austenite. Dynamically recrystallized austenitic grains were strongly coarsened with decreasing strain rate and growing temperature. For the austenitic region, the relationship between the peak strain and the Zener–Hollomon parameter has been derived, and the phenomenological constitutive model describing the flow stress depending on temperature, true strain rate and true strain was developed. The model can be used to predict the forming forces in the seamless tubes production of the given steel.

Microstructural changes around grain boundary after long-term creep in low-alloy steels

Microstructural changes after long-term creep were investigated in low-alloy Cr–Mo steels, focusing on the precipitation-free zone (PFZ) around the grain boundary. The grip and gauge portions of creep-ruptured samples obtained through the NIMS Creep Data Sheet Project were observed by SEM. The PFZ was clearly observed around the grain boundary after long-term creep in 1Cr-0.5Mo (JIS STBA22) and 1.25Cr-0.5Mo–Si (JIS STBA23) steels. Large precipitates were formed on the grain boundaries near where the PFZ was observed. The width of the PFZ increased with increasing creep time at 550 °C–650 °C. The increase in width was remarkable at higher temperatures. The clear relationship between the PFZ width and the Larson-Miller parameter (LMP) has already been reported for 2.25Cr–1Mo steel. This relationship provides a method for predicting the metal temperatures of serviced components in power plants. A good relationship between the PFZ width and the LMP was confirmed for 1Cr-0.5Mo and 1.25Cr-0.5Mo–Si steels. However, the relationship was not consistent with that for 2.25Cr–1Mo steel. The increase in the PFZ width was faster in 1.25Cr-0.5Mo–Si steel than in 1Cr-0.5Mo steel.

The Investigation of the Influence of Modern Coating Applied to the Cutting Inserts During Machining

This article deals with benefits of PVD and CVD coatings applied to cutting inserts during face milling technology. The coatings deposited on applied cutting inserts were used to extension the tool life. Machining process was carried out on the conventional vertical milling machine tool. This machining process was performed without coolant. The set up of cutting conditions were constant throughout the machining and low-alloy Cr-Mo steel DIN 42CrMoS4 (W. Nr. 1.7227) was used as workpiece material in the realized process of experiments. The aim of this investigation was to compare coated and uncoated changeable cutting inserts clamped in the milling cutter and find out the benefits of PVD or CVD coatings during realization of face milling process. The monitored parameters were the force load measured by piezoeletrical dynamometer Kistler 9257B and the obtained flank wear measured by optical microscope.

Effect of tempering heat treatment on the CO2 corrosion resistance of quench-hardened Cr-Mo low-alloy steels for oil and gas applications

The corrosion resistances of a number of tempered states of two Cr-Mo low-alloy steels, the novel “HT10″ (martensitic) and the commercial ASTM A182 grade “F22″ (bainitic), were evaluated in CO2-saturated brine. The tempering heat treatments soften both steels, due to a decrease in dislocation density, and the precipitation of alloy carbides, but also decrease their uniform corrosion rates. Such a decrease is considerable if the steels are tempered at higher temperature for longer time. For the novel HT10, we discuss how the tempering-induced changes to the microstructure contribute to decreasing the cathodic reaction kinetics, and thus the general corrosion rate.

Effect of Nb on improving the impact toughness of Mo-containing low-alloyed steels

The microalloying of low-alloyed steels with Nb can improve the strength-to-toughness balance. Such an effect of Nb is usually ascribed to the refinement of the grain structure occurring in the austenite regime during hot forming. In the present work, we report that Nb enhances the impact toughness of a low-alloyed Cr–Mo steel by a mechanism which has not been appreciated so far. The lower impact toughness in the Nb-free Cr–Mo steel is due to segregation of Mo to boundaries, which facilitates the formation of fine Mo-rich ξ-phase carbides lining up along the boundaries. This further promotes the nucleation and propagation of microcracks. The addition of Nb leads to the formation of Mo-enriched NbC particles. The interfaces between these particles and the matrix supply new preferential sites for precipitation of Mo-rich ξ-phase carbides upon subsequent tempering. In this way, Nb additions result in a decrease of Mo segregation to boundaries, significantly reducing the precipitation of ξ-phase carbides on grain boundaries, thus leading to improved impact toughness. In addition to the classical microstructural explanation (grain size effect), this chemical role of Nb sheds new light on the design strategies of advanced low-alloyed steels with optimized strength-to-toughness ratios.

Risk based analysis of temperature and time effects on brittle fracture of A-387 Gr. B welded joint

Risk based analysis of temperature and time effects on fracture toughness values is applied to different regions of a welded joint made of low-alloyed Cr-Mo steel A-387 Gr. B, designed for high temperature applications. Heterogeneity of microstructure and properties of welded joint is evaluated by testing standard 3BP specimens with crack tip located at different regions of a joint, including the base metal (BM), weld metal (WM) and heat-affected-zone (HAZ). Experiments were performed both at the room temperature and at design working temperature, 540°C. Based on these results, fracture toughness values are determined and used for risk based analysis, including risk matrix presentation as the basis for decision making process.

Influence of temperature on fracture toughness values in different regions of A-387 Gr. B welded joint

The influence of temperature on fracture toughness values in different regions of a welded joint is analysed low-alloyed Cr-Mo steel A-387 Gr. B, designed for high temperature applications. Heterogeneity of microstructure and properties of welded joint is evaluated by testing standard 3BP specimens with crack tip located at different regions of a joint, including the base metal (BM), weld metal (WM) and heat-affected-zone (HAZ). Experiments were performed both at the room temperature and at design working temperature, 540°C. Based on these results, temperature effect on crack resistance is established for all different regions in a welded joint.

Relation between impact and fracture toughness of A-387 Gr. B welded joint

The influence of temperature on impact and fracture toughness values in different regions of a welded joint is analysed for low-alloyed Cr-Mo steel A-387 Gr. B, designed for high temperature applications. Standard Charpy specimens were tested on instrumented pendulum to separate total impact energy into energy for initiation and propagation energy for base metal (BM), weld metal (WM) and heat-affected-zone (HAZ). Standard three point bending (3BP) specimens with crack tip located at different regions of a joint (BM, WM, HAZ), were used for fracture toughness testing. Experiments were performed both at the room temperature and at design working temperature, 5400 C, which is the focus of this paper, to evaluate temperature effect on both notch and crack resistance for all different regions in a welded joint. Moreover, the relation between crack initiation energy and fracture toughness is established, purely on empirical base.

Fatigue limit of carbon and Cr[sbnd]Mo steels as a small fatigue crack threshold in high-pressure hydrogen gas

The fatigue limit properties of a carbon steel and a low-alloy CrMo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the CrMo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained SN data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.

The impact of the temperature and exploitation time on the tensile properties and plain strain fracture toughness, KIc in characteristic areas of welded joint

This paper presents the analysis of the temperature and exploitation time impact on the resistant measure to brittle fracture of welded joint constituents of the new and exploited low-alloyed Cr-Mo steel A-387 Gr. B from the aspect of application of the parameters obtained by tensile testing and parameters obtained by fracture mechanics testing. The exploited parent metal is a part of the reactor mantle which has been exploited for over 40 years and is in the damage repair stage, i.e. in the stage where the part of the mantle is being replaced with new material. Basic characteristics of the material strength, as well as the stress-elongation curves required for stress analysis are obtained by tensile testing. The testing of plane strain fracture toughness is conducted in order to determine the critical stress intensity factor, KIc, that is, assessment of behavior of the new and exploited parent metal, welded metal and heat affected zone from the side of the new parent metal and from the side of the exploited parent metal in the presence of the crack type error. Based on the research results, the analysis of the resistance to brittle fracture represents the comparison of obtained values for characteristic areas of welded joint and justification of the selected welding technology.

Influence of Thermal History on the Hot Ductility of a Continuously Cast Low Alloyed Cr-Mo Steel

The hot ductility of a low alloyed Cr-Mo steel has been investigated to evaluate the surface cracking sensitivity within the straightening or unbending regime during the continuous casting process. Tensile samples were subjected to various thermal treatments, including melting and solidification, and were tested at deforming temperatures ranging between 600 and 1100 °C using a strain rate of 10−3 s−1. Hot ductility was evaluated based on reduction in area measurement and metallographic investigations. The investigated steel exhibits a drop in ductility at around 800 °C due to intergranular cracking. Microstructural examinations and supplementary thermokinetic computer simulations were carried out to describe the evolution of the microstructure during solidification and cooling.

Hardening Embrittlement and Non-Hardening Embrittlement of Welding-Heat-Affected Zones in a Cr-Mo Low Alloy Steel

The embrittlement of heat affected zones (HAZs) resulting from the welding of a P-doped 2.25Cr-1Mo steel was studied by the analysis of the fracture appearance transition temperatures (FATTs) of the HAZs simulated under a heat input of 45 kJ/cm with different peak temperatures. The FATTs of the HAZs both with and without tempering increased with the rise of the peak temperature. However, the FATTs were apparently lower for the tempered HAZs. For the as-welded (untempered) HAZs, the FATTs were mainly affected by residual stress, martensite/austenite (M/A) islands, and bainite morphology. The observed embrittlement is a hardening embrittlement. On the other hand, the FATTs of the tempered HAZs were mainly affected by phosphorus grain boundary segregation, thereby causing a non-hardening embrittlement. The results demonstrate that the hardening embrittlement of the as-welded HAZs was more severe than the non-hardening embrittlement of the tempered HAZs. Consequently, a post-weld heat treatment should be carried out if possible so as to eliminate the hardening embrittlement.

Effect of temperature and exploitation time on tensile properties and plain strain fracture toughness, KIc, in a welded joint

Effect of temperature and exploitation time on resistance to brittle fracture of welded joint constituents of new and 40 years exploited low-alloyed Cr-Mo steel A-387 Gr. B has been analysed. The plane strain fracture toughness is used as the measure of resistance to brittle fracture of base metal, weld metal and heat affected zone from both new and exploited. The stress-strain curves have been determined as well, to get better inside in welded joint behavior. Comparison of the results indicate detrimental effect of both temperature and exploitation time, both for tensile properties and for the resistance to brittle fracture.