Effect of Heat Treatment on Microstructure and Mechanical Properties of 2.25Cr–1Mo Steel

The effect of heat treatment on microstructure and mechanical properties of a ferritic heat‐resistant steel, 2.25Cr–1Mo, was investigated. The characteristics of carbides, i.e., size, morphology, distribution, and composition, in different stages of heat treatment were examined by using transmission electron microscopy (TEM) and energy dispersive X‐ray (EDX). As the heat treatment proceeded from normalizing and tempering (NT, the delivery state), simulated post‐welding heat treatment (PWHT), to step cooling (SC), the carbides evolved from M₃C, Mo₂C to M₇C₃, M₂₃C₆. The uniformly dispersed carbides, e.g., Mo₂C, M₇C₃, and M₂₃C₆, are responsible for the excellent mechanical properties and creep resistance of the steel. The result demonstrates that the coarsening of carbides deteriorates impact toughness and the morphology, crystal structure, and chemical composition of carbides are also important influencing factors.     

Present quality level of large heat resistant rotor forgings made of 1% CrMoV steel – Part II

While in part I mainly mechanical and toughness properties together with structural aspects have been dealt with now design properties are described. The basic properties for heat resistant rotor materials are traditionally the creep properties. So they are here investigated more in detail and it is shown that modern 1% CrMoV rotor steels have a quite similar creep behaviour both oil and air quenched. Long term high temperature exposure and its influence on the properties are as well mentioned as heat stability behaviour.     

Effect of carbon content in CrMoV hot working tool steel

The aim of this work was to find out whether a reduction of carbon content would reduce the amount of segregated eutectic carbides and improve ductility and toughness. Quenched and temperted specimens of steel X 40 CrMoV 5 1 (～AISI H 13) were tested at carbon contents of 0.42 and 0.27%. Tensile and short time creep tests were carried out at elevated temperatures. Toughness was derived from unnotched, notched and precracked specimens over a temperature range. Stable fatigue crack growth was measured in CT specimens at room temperature and in thermal fatigue specimens. As expected, ductility and toughness are raised by reducing the carbon content. At the same time a favourable improvement of hot strength and creep resistance appears, causing a lower thermal fatigue crack velocity. These beneficial effects are due to the higher alloy content in solution after hardening. It is proposed to test a steel X 28 CrMoV 5 1 for intricate tools in practical application and learn more about its wear behaviour.     

Microstructure and Properties of Hot Working Die Steel H13MOD

Compared with H13 steel, the influences of different heat treatment process on the microstructure and properties of the new type of hot working die steel H13MOD were studied. The results show that the complete austenitizing temperature of H13MOD is around 1030 °C and the quenching hardness achieves the maximum value at this temperature. While for H13, the complete austenitizing temperature is above 1100 °C and the quenching hardness rise constantly with the quenching temperature increasing. In quenching process, the undissolved MC carbides can prevent the coarsening of grain in both steels. With the rise of quenching temperature, when MC carbides dissolve completely, the grain grows quickly. The hardness and strength of H13MOD at higher tempering temperature (above 570 °C) are nearly the same as those of H13, but its toughness is higher than that of H13. Mo₂ C carbide is the main strengthening phase in H13MOD, which is attributed to the higher content of Mo. The quantity of VC eutectic carbides is reduced because of lower content of V in H13MOD, which plays an important role in enhancing the impact toughness of H13MOD. Under a certain strength condition, H13MOD steel can be used in the environment that higher toughness is required and the service life of die casting mold can be improved.     

Microstructural changes in a 12% chromium steel during creep

A quantitative metallographic study was performed using transmission electron microscopy (TEM) to describe the microstructural changes in a 12% chromium steel (X 20 CrMoV 12 1) during creep at 650°C. The creep experiments were conducted under constant load conditions corresponding to initial stresses of 175 and 80 N/mm². The heat treatment for this steel consists of austenitizing followed by tempering which results in a high density of free dislocations within small elongated subgrains with carbides on or very near some of the subgrain boundaries. During creep, the mean subgrain size increases for both the high and low stress levels. Carbide particle coarsening is observed for the low stress level. These processes result in a softening of the microstructure during creep deformation.     

The effect of titanium on creep strength in 2.25 Pct Cr-1 Pct Mo steels

The effect of a low level of titanium on the microstructure and creep properties of 2.25 pct Cr-1 pct Mo steels has been examined as a function of carbon content and austenitizing temperature. The addition of 0.04 wt pct titanium resulted in a dramatic increase in creep strength at 565 °C, and this was found to be associated with the presence in the microstructure of very small (50 to 100 Å) titanium-bearing precipitates based upon both TiC and Mo₂C. The variation of the minimum creep rate with carbon content and austenitizing treatment was explained in terms of the solubility of TiC in austenite. The titanium-bearing carbides have an important effect on microstructural stability and on the maintenance of creep strength, but it is also apparent that solid solution strengthening by molybdenum can make a significant contribution to creep strength at low carbon levels (0.02 wt pct).     

Microstructure and deformation rate during long-term cyclic creep of the martensitic steel X22CrMoV12-1

The microstructure of three specimens of the martensitic steel X22CrMoV12-1 which had been subjected to long-term cyclic creep at 873 K with intermittent phases of unloading (stress ratio R = 0) and compression (R = ?1) was quantified by electron microscopy with regard to carbides, dislocations and pores. The laws of time dependent coarsening of carbides and strain controlled growth of subgrains found for monotonic creep hold also for cyclic creep. The longer time it takes cyclic creep to reach a given strain leads to a growth advantage of carbides compared to monotonic creep. The microstructural model of plastic deformation previously developed for monotonic creep on X20(22)CrMoV12-1 allows to calculate the cyclic creep acceleration due to this advantage in carbide growth.     

Influence of Heat Treatment Parameters on the Carbide Morphology of PM High-Speed Steel HS 6-5-3-8

The influence of heat treatment parameters on the carbide morphology of the powder metallurgic high-speed steel HS 6-5-3-8 is examined. To that end, diverse heat treatment parameters are selected and applied by quenching dilatometry. In particular, different austenitizing temperatures, as well as an isothermal holding stage during quenching in the temperature regime of the transformation gap at temperatures between 450 and 600 °C, are produced. Extensive computer-aided image analysis is performed to investigate the carbide morphology. It is found that the circularity of the tungsten-rich M₆C carbides increases significantly after short holding times at a temperature of 550 °C due to the carbide precipitation from metastable and supersaturated austenite onto pre-existing carbides. Longer holding times lead to further growth of the carbides, while the circularity of the carbides does not change. It is further shown that the hardness of the isothermally treated material is increased, all other parameters being equal. Increased carbide circularity might be helpful for increasing the toughness while reaching the same hardness and wear resistance as the conventional heat-treated material. Moreover, it might be possible to enhance the austenitizing temperature with regain of positive carbide morphology properties during the isothermal holding stage. Thus, improved material properties could be achieved.     

Microstructure of a Creep-Resistant 10 Pct Chromium Steel Containing 250 ppm Boron

The microstructure of a trial martensitic chromium steel containing a high content of boron (250 ppm) was characterized in detail in the as-tempered and aged conditions. This steel has a similar composition and heat treatment as the TAF steel that still is unsurpassed in creep strength among all 9 to 12 pct chromium steels. Characterization was performed by using scanning electron microscopy, energy-filtered transmission electron microscopy, secondary ion mass spectroscopy, and atom probe tomography. Focus was placed on investigating different types of precipitates that play a key role in improving the creep resistance of these steels. The low tempering temperature of 963 K (690 °C) is enough for the precipitation of the full volume fraction of both MX and M₂₃C₆. A high boron content, more than 1 at. pct, was found in M₂₃C₆ precipitates and they grow slowly during aging. The high boron level in the steel results in metal borides rather than BN with the approximate formula (Mo_0.66Cr_0.34)₂(Fe_0.75V_0.25)B₂. Two families of MX precipitates were found, one at lath boundaries about 35 nm in size and one dense inside the laths, only 5 to 15 nm in size.     

How TEM Projection Artifacts Distort Microstructure Measurements: A Case Study in a 9 pct Cr-Mo-V Steel

Morphological data obtained from two-dimensional (2D) and three-dimensional (3D) transmission electron microscopy (TEM) observations were compared to assess the effects of TEM projection errors for submicron-size precipitates. The microstructure consisted of M₂₃C₆ carbides in a 9 pct Cr-Mo-V heat resistant steel before and after exposure to creep conditions. Measurements obtained from about 800 carbides demonstrate that particle size and spacing estimates made from 2D observations overestimate the more accurate values obtained from 3D reconstructions. The 3D analysis also revealed the M₂₃C₆ precipitates lengthen anisotropically along lath boundary planes, suggesting that coarsening during the early stage of creep in this alloy system is governed by grain boundary diffusion.     

回火温度对轧后直接水淬15CrMoV钢组织和力学性能的影响

试验用钢15CrMoV(%:0.15C、0.29Si、0.57Mn、1.01 Cr、0.37Mo、0.24V)16 mm板材的终轧温度为900～950℃,轧后在880～900℃水淬,并经670～800℃回火。结果表明,试验钢在线淬火后的组织为马氏体+贝氏体,随回火温度升高,钢中碳化物析出量增加,贝氏体板条束逐渐合并和减少,最终转化为碳化物+多边形铁素体组织;在730～780℃回火,15CrMoV钢具有良好的综合力学性能,抗拉强度680～760 MPa,冲击功55～130 J。     

Development and Application of a Stainless CrNbMoC Tool Steel

Steels with 15 mass% Cr, 2 mass% Mo and varying contents of C, V, Nb and Ti were investigated by thermodynamic calculations to find a stainless grade promising a wear resistance equal to that of the standard cold work tool steel X153CrMoV12. It was shown that Nb is most suited to form MC carbides for wear resistance thus reducing the content of M₇C₃, which in turn raised the Cr content dissolved in the matrix to a passivating level. Small melts in the vicinity of steel X140CrNbMoTi15‐5‐2 confirmed this concept in respect to hardenability, wear resistance and corrosion resistance. Industrial manufacturing and application of the new grade RN15X® will be discussed.     

Review of research developments of 12Cr1MoVG heat resistant steel in China

In this paper,we review the evolution of 12CrlMoV steel standards at home and abroad,analyze the effects of various elements and determine their optimal contents in steel.The influence of heat treatment and microstructure on the creep strength of 12Cr1MoV steel is investigated.Statistical results from conventional mechanical properties,ductile-brittle transition temperature,high temperature oxidation resistance,aging,instant high temperature properties,and creep are introduced.The results show that the chemical composition and heat treatment process of 12CrlMoVG steel identified in the GB 5310-2008 standard is appropriate,resulting in a steel with higher creep strength and good comprehensive properties.     

Short-Term Heat Treatment of the High-Alloy Cold-Work Tool Steel X153CrMoV12: Calculation of Metastable Microstructural States

The influence of short-time heat treatment on the widely used and commercially available ledeburitic cold-work tool steel 1.2379 (X153CrMoV12; AISI D2) is examined herein. Starting from a soft annealed initial condition, the influence of different austenitizing temperatures and holding times on the metastable microstructural states after heat treatment/hardening is investigated. The experimental implementation of the heat treatment is used in a quenching dilatometer, and a microstructural simulation model is built using these results. As validation of the model, on the one hand, the martensite start temperature (M_s) is used, measured experimentally by dilatometry. Additionally, the carbide content and distribution, as determined by quantitative image analysis, are compared with the simulated data and used as an indicator of the model accuracy. Through the developed simulation model, arbitrary heat treatment-induced metastable microstructural states can be calculated. As a possible application of this model, the live-adaption of the industrial heat treatment process in dependence on the batch chemical composition is discussed.     

Effects of alloying elements on fracture toughness in the transition temperature region of base metals and simulated heat-affected zones of Mn-Mo-Ni low-alloy steels

This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment, and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and increased Mo and Ni content, the number of fine M₂C carbides was greatly increased and the number of coarse M₃C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to precipitate fine M₂C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base metals.     

Service Reliability Assessment of Weld Repaired CrMoV Steel Rotors

This research deals with the service evaluation of a retired CrMoV steel rotor that has been repaired using deep groove multi‐pass submerged arc welding (SAW) and gas tungsten arc welding (GTAW). Accelerated creep tests were conducted at several elevated temperatures and extrapolated to the service temperatures. The coalescence of carbides and cavity growth during creep were evaluated. For the CrMoV‐GTAW samples, it was found that the creep life depended on the location of the crossweld in the welded specimen. The service lives of the weldments were extrapolated.     

Thermal Fatigue Testing of Plasma Transfer Arc Stellite Coatings on Hot Work Tool Steels under Steel Thixoforming Conditions

The thermal fatigue performance of Stellite 12 coating deposited on X32CrMoV33 hot work tool steel via the plasma transfer arc (PTA) process was investigated under steel thixoforming conditions. Stellite 12 coating has made a favorable impact on the thermal fatigue performance of the X32CrMoV33 hot work tool steel. The latter survived steel thixoforming conditions lasting much longer, for a total of 5000 cycles, when coated with a PTA Stellite 12 layer. This marked improvement is attributed to the higher resistance to oxidation and to temper softening of the Stellite 12 alloy. The Cr-rich oxides, which form during thermal cycling, provide adequate protection to high-temperature oxidation. In contrast to hot work tool steel, Stellite 12 alloy enjoys hardening upon thermal exposure under steel thixoforming conditions. This increase in the strength of the coating is produced by the formation of carbides and contributes to the superior thermal fatigue resistance of the Stellite 12 alloy. When the crack finally initiates, it propagates via the fracture of hard interdendritic carbides. The transformation of M₇C₃ to M₂₃C₆, which is more voluminous than M₇C₃, promotes crack propagation.     

热处理工艺对20Cr1Mo1VTiB螺栓钢持久性能的影响

 为了更好地验证20Cr1Mo1VTiB螺栓钢在国家标准和俄罗斯标准热处理工艺下的持久性能,采用SEM、TEM等手段研究了两种标准的热处理工艺对20Cr1Mo1VTiB螺栓钢组织及持久性能的影响。结果表明,相对于低温淬火+低温回火(俄罗斯标准)工艺,高温淬火+高温回火(国家标准)的热处理工艺生产的20Cr1Mo1VTiB具有更高的持久性能稳定性。俄罗斯标准工艺淬火温度低,未溶的块状富钛碳化物数量多、尺寸大,并且贝氏体内分布大量方(菱)形TiC,促进了蠕变孔洞的形成,因此持久强度下降速率更快。国家标准工艺淬火温度高,富钛碳化物回溶充分,数量少、尺寸小,回火过程析出细小的颗粒状VC,热稳定性较高,具有更高的持久强度。从持久性能角度,选用国家标准工艺热处理更加合理。     

The evaluation of in-service materials degradation of low-alloy steels by the electrochemical method

The nondestructive evaluation procedure for detecting in-service materials degradation of low-alloy 2.25Cr-1Mo and CrMoV steels by the electrochemical method has been investigated. The results can be summarized as follows. (1) For 2.25Cr-1Mo steels, the peak current mainly caused by the selective dissolution of coarse carbides M₆C appears at ∼+100 mV during potentiodynamic polarization measurements in dilute sodium molybdate solution. This peak value of current density, ΔI_p, can be chosen as a reflective parameter of an amount of coarse carbides M₆C and shows excellent correlations both with shifts in fracture appearance transition temperature (FATT) caused by carbide coarsening and with hardness change. Actual operational temperature can be estimated from operational period, since the Larson-Miller time-temperature parameter (LMP) value of materials has a unique relationship with ΔI_p values. (2) For CrMoV steels, the evaluation of temper embrittlement of CrMoV cast steel by a novel electrochemical technique is described. Intergranular corrosion (IGC) occurs only on temper-embrittled samples during anodic polarization process in calcium nitrate solution. The characteristic changes in polarization curves attributed to IGC have an excellent correlation with shifts in FATT caused by temper embrittlement.     

Study on microstructure and mechanical properties of martensitic stainless steel 6Cr15MoV

Martensitic stainless steel containing 12%-18%Cr have high hardness due to high carbon content. These steels are common utilized in quenching and tempering processes for knife and cutlery steel.The properties obtained in these materials are significantly influenced by matrix composition after heat treatment,especially as Cr and C content.Comprehensive considered the hardness and corrosion resistance,a new type martensitic stainless steel 6Cr15MoV has been developed.This study emphatic researches the effect of heat treatment processes on microstructure and mechanical properties of 6Cr15MoV martensitic stainless steel.Thermo-Calc software has been carried out to thermodynamic calculation;optical microscope（OM）,scanning electronic microscope（SEM） and transmission electron microscope（TEM） have been carried out to microstructure observation;hardness and impact toughness test have been carried out to evaluate the mechanical properties.Results show that the equilibrium carbide in 6Cr15MoV steel is M₂₃,C₆ carbide,and finely distributed of M₂₃C₆ carbides can be observed on annealed microstructure of 6Cr15MoV stainless steel.6Cr15MoV martensitic stainless steel has a wider quenching temperature range,the hardness value of steel 6Cr15MoV can reach to 60.8 -61.6 HRC when quenched at 1060 - 1100℃.Finely distributed carbides will exist in quenched microstructure,and effectively inhabit the growth of austenite grain.With the increasing of quenching temperature,the volume fraction of undissolved carbides will decrease.The excellent comprehensive mechanical properties can be obtained by quenched at 1060-1100℃with tempered at 100-150℃,and it is mainly due to the high carbon martensite and fine grain size.At these temperature ranges,the hardness will retain about 59.2-61.6 HRC and the Charpy U-notch impact toughness will retain about 17.3-20 J.The morphology of impact fracture surface of tested steel is small dimples with a small amount of cleavage planes.The area of cleavage planes increases with the increasing of tempering temperature.