Temper embrittlement of hot work die steel

Abstract

The influence of the parameters of an additional tempering at 600°C on temper embrittlement in quenched and previously double tempered 5 wt-%Cr tool steel having a post-martensitic microstructure was investigated. A detailed examination of associated changes of fracture mechanism, microstructure, and precipitate was carried out. The dominant factor controlling the evolution of brittleness was the segregation of phosphorus to prior grain and lath boundaries as well as to martensite lath/carbide interfaces. The segregation effect of phosphorus at these interfaces was accompanied by cosegregation of sulphur and silicon together with simultaneous carbide precipitation.

MST/1132     

Structure Character of M-A Constituent in CGHAZ of New Ultra-Low Carbon Bainitic Steel under Laser Welding Conditions

800 MPa grade new ultra-low carbon bainitic （NULCB） steel is the recently developed new generation steel. The microstructure in the coarse-grained heat affected zone （CGHAZ） of NULCB steel under laser welding conditions was investigated by thermal simulation. The influence of the cooling time from 800℃ to 500℃.t8/5 （0.3-30 s）, on the microstructure of the CGHAZ was discussed. The experimental results indicate that the microstructnre of the CGHAZ is only the granular bainite which consists of bainitic ferrite （BF） lath and M-A constituent while t8/5 is 0.3-30 s. The M-A constituent consists of twinned martensite and residual austenite, and the change of the volume fraction of the residual austenite in the M-A constituent is very small when t8/5 is between 0.3 and 30 s. The morphology of the M-A constituent obviously changes with the variation of t8/5.As t8/5 increases, tile average width, gross and shape parameter of the M-A constituent increase, while the line density of the M-A constituent decreases.     

Effect of weld heat input on toughness and structure of HAZ of a new super-high strength steel

Fracture morphology and fine structure in the heat-affected zone (HAZ) of HQ130 super-high strength steel are studied by means of SEM, TEM and electron diffraction technique. Test results indicated that the structure of HAZ of HQ130 steel was mainly lath martensite (ML), in which there were a lot of dislocations in the sub-structure inside ML lath, the dislocation density was about (0.3∼ 0.9) x 10¹²/cm². No obvious twin was observed in the HAZ under the condition of normal weld heat input. By controlling weld heat input (E ≤20 kJ/cm), the impact toughness in the HAZ can be assured.     

Effects of Carbon on the CG HAZ Toughness and Transformation of X80 Pipeline Steel

X80 pipeline steel produced by TMCP has high strength and high toughness with ultrafine grain microstructure. The microstructure coarsens and the toughness worsens at the coarse grained (CG) HAZ apparently after weld simulation. The experimental results indicated that the bainitic ferrite and the second phases formed at cooling are differently as the variation of carbon in base metal. In low carbon steels, the bainitic ferrite laths are long and narrow, the second phases are complex including residual austenite, martensite, the M-A constituent and the Fe3C carbide. The formation of Fe3C carbide is the main reason of the poor toughness in CG HAZ. The ultralow carbon in base metal, however, can improve the CG HAZ toughness through restraining the formation of carbides, decreasing the M-A constituent, increasing the residual austenite content, which are beneficial to the CG HAZ toughness.     

X90钢级管线钢预精焊接头的性能研究

为综合研究X90管线钢的焊接性,选用国内某钢厂轧制的X90管线钢卷板,利用预精焊工艺制备试验钢管4根,采用金相分析、扫描电镜(SEM)断口分析、夏比V型缺口冲击试验、拉伸、弯曲、硬度等试验,研究了焊接接头各个区域的组织和性能.试验结果表明:内外焊缝区组织均为针状铁素体,热影响区(HAZ)粗晶区晶粒粗化严重,主要组织为粒状贝氏体和贝氏体铁素体,在原奥氏体晶界和贝氏体板条内部存在块状或条状的(马氏体-奥氏体)M-A组元;HAZ冲击功离散性较大,出现了单值较低(45 J)的试样,SEM断口分析呈现典型的解理断裂特征;焊接接头抗拉强度805~815 MPa,断裂位置均在HAZ;焊接接头反弯试样易在HAZ出现裂纹和脆断现象;HAZ硬度在220~250 HV之间,较母材下降30 HV左右.HAZ是X90预精焊钢管焊接接头的薄弱环节,为提高X90管线钢的焊接稳定性,应重点研究精焊内外热循环双热影响亚区的组织转变和脆化机理.     

Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 1 – Strip processing

Abstract

Simulations of industrial thermomechanical processing and on line accelerated cooling of a low carbon microalloyed strip steel were carried out using a quench deformation dilatometer. Effects of processing parameters, such as accelerated cooling rate T and accelerated cooling interrupt temperature T_I on the critical transformation temperatures and final microstructure were determined. The most important on line accelerated cooling (OLAC) processing parameter is the accelerated cooling interrupt temperature, which controls whether the transformed microstructure is predominantly ferrite or bainite. A variety of (Ti, Nb, Fe) carbide, nitride, and carbonitride precipitates are present in the OLAC processed samples. The final precipitate distribution is developed at three stages of processing, namely: dissolution and coarsening of pre-existing precipitates at the reheat temperature, precipitation in deformed austenite during the deformation schedule, and precipitation in ferrite after the interruption of accelerated cooling. Maximum precipitation strengthening occurs for T_I=700–640°C.

MST/3424     

Microstructure, Impact Toughness and TEM Analysis in the CGHAZ of HQ130 High Strength Steel

1.IntroductionThedifferentpartsintheheat-affectedz0neofthehighstrengthsteelexperienceddifferentweldthermalcycles,inwhichthechangeofmicrostructureandper-formanceinthecoarsegrainheat-affectedzone(CG-HAZ)nearthefusionzoneismost0bvious,andcoldcrackingiseasytobeproducedandspreadinthiszone.CGHAZwasconsideredasone0fthem0stweakpositioninwholeweldingjoilltzone,andllu-merousinvestigatorshavepaidcl0seattentiontotheCGHAZl1'2].HQ130steel,whichhastensilestrength(UTs)ofmorethan1300MPa,isanewlydevelop…     

Effects of co-addition of titanium and boron on microstructure of superplastic Cr–Mo steels

Abstract

The effects of titanium and boron on the microstructure of a low alloyed Cr–Mo steel with 0·6 wt-%C have been investigated by comparison with a steel containing only titanium and a steel free from both titanium and boron. Each of the steels was subjected to thermomechanical treatment and annealed at 700°C, resulting in small grains of size a few micrometres. The steel containing both titanium and boron possessed the smallest ferrite grains and M₃C carbides of the three examined. This is attributed to a fine dispersion of borides (TiB₂ ) and borocarbides (Ti(C,B)) of size 10 nm in the ferrite matrix through the pinning effect. At the grain boundaries small carbide particles were present which were effective in inhibiting grain boundary migration. The extremely fine borides and/or borocarbides were useful in suppressing intragranular deformation of ferrite grains due to precipitation hardening. This may have assisted in promoting grain boundary sliding, resulting in superior superplastic elongation.     

A Study on the Effect of Thermal Ageing on the Specific-Heat Characteristics of 9Cr–1Mo–0.1C (mass%) Steel

The effect of thermal ageing on the heat-capacity behavior of 9Cr–1Mo–0.1C (mass%) ferritic/martensitic steel has been studied using differential scanning calorimetry (DSC) in the temperature range from 473 K to 1,273 K. The DSC results in the case of slow cooled, normalized and tempered, and subsequently thermally aged samples (500 h to 5,000 h at 823 K (550 °C) and 923 K (650 °C), clearly marked the presence of both magnetic and α-ferrite + carbide → γ-austenite phase transformations that take place successively upon heating. Furthermore, for the case of fully martensitic microstructure, an additional exothermic transformation at about 920 K(647 °C), arising from carbide precipitation is noticed. This event is characterized by a sharp drop in C _P . It is found that the α-ferrite + carbide → γ-austenite phase transformation temperature is only mildly sensitive to microstructural details, but the enthalpy change associated with this phase transformation, and especially the change in specific heat around the transformation regime, are found to be dependent on the starting microstructure generated by thermal ageing treatment. Prolonged ageing for about 500 h to 5,000 h in the temperature range from 823 K to 923 K (550 °C to 650 °C) contributed to a decrease in heat capacity, as compared to the normalized and tempered sample. This is due to the increase in carbide volume fraction. The martensitic microstructure is found to possess the lowest room-temperature C _P among different microstructures.     

Effect of austenite transformation degree on microstructure and fracture toughness of high-strain pipeline steel

In order to clarify the relationships among austenite transformation degree, microstructure and fracture toughness, the simulated inter-critical heat-affected zones (ICHAZ) of high-strain pipeline steel were prepared with five different austenitizing degrees (0%, 20%, 50%, 80% and 100%). The microstructure and the fracture toughness of simulated ICHAZ specimens were investigated. It was found that the microstructure evolution and fracture toughness of ICHAZ were closely related to the austenite transformation degree. In partially austenitized ICHAZ, the fresh bainite and ferrite could break the original structure and decreased effective grain size. The fine grains with disorderly arranged M–A constituents could improve fracture toughness of partially austenitized ICHAZ. In contrast, the linearly aligned M–A constituents in 0%-austenitized region and the chain-distributed ones in 100%-austenitized region could lead to deterioration of fracture toughness. Furthermore, the excellent fracture toughness of partially austenitized ICHAZ was related to the high density of high-angle grain boundaries (HAGBs), homogeneous distribution of local strain as well as the high percentage of small-deformed grains (SDGs).

     

Effect of molybdenum on microstructure and wear properties of Fe–Ti–Mo–C laser clad coatings

Abstract

The AISI?1045 steel surface was alloyed with preplaced ferrotitanium (Fe–Ti), ferromolybdenum (Fe–Mo) and graphite powders using a 5 kW CO₂ laser. In situ carbide reinforced Fe based surface composite coating was fabricated. The results showed that (Ti,Mo)C particles with flower-like and cubic shapes were formed during laser cladding process. The growth morphology of the reinforcing (Ti,Mo)C carbide has typically faceted features, indicating that the lateral growth mechanism is still the predominant growth mode under rapid solidification conditions. Increasing the amount of Fe–Mo in the reactants led to a decrease in carbide size and an increase in volume fraction of carbide but increased the crack sensitivity of the coating. The multiple carbides of (Ti,Mo)C created a higher microhardness and excellent wear resistance than TiC alone under dry sliding wear test condition.     

Tensile fracture behaviour of microstructurally engineered Cu bearing high strength low alloy steel

Abstract

An attempt has been made to highlight the influence of precipitation and microstructural constituents on tensile fracture behaviour in Cu bearing HSLA 100 steel. Variations in the microconstituents have been incorporated in the steel by engineering the microstructures through thermal treatments consisting of solutionising, water quenching and aging at various temperatures. The microstructure in quenched condition consists of mainly lath martensite, bainite and acicular ferrite besides little amount of retained austenite, carbides and carbonitrides. Aging up to 500°C facilitated fine coherent ?-Cu precipitation that lost its coherency at >550°C. Simultaneously, recovery and recrystallisation of martensite and acicular ferrite occurred at higher temperatures. The formation of new martensite islands occurred on aging at >650°C. Carbides, carbonitrides and retained austenite remained essentially unchanged. Tensile tests were conducted at a slow strain rate to study the tensile fracture behaviour of the steel. Microstructural and fractographic evidences indicating that coherent Cu precipitate causes the brittleness in the material in initial stages of aging whereas loss of coherency of Cu precipitate in later stages results in the reappearance of ductility in the material.     

Fracture toughness of steels with nickel content in respect of carbide morphology

Abstract

This paper is focused on the influence of Ni addition on the microstructure and fracture toughness of structural steels after tempering. Nickel is known to increase the resistance to cleavage fracture of steel and decrease a ductile–brittle transition temperature. The medium carbon, low alloy martensitic steels attain the best combination of properties in low tempered condition, with tempered martensite, retained austenite and transition carbides in the microstructure. In the present research, four model alloys of different Ni contents (from 0·35 to 4·00%) were used. All samples were in as quenched and tempered condition. Quenching was performed in oil at room temperature. After quenching, samples were tempered at 200°C for 2?h. An increase in nickel content in the investigated model structural steels causes a decrease in ε carbide volume fraction in their microstructure. Cementite nucleates independently in the boundaries of martensite laths and in the twin boundaries in the areas where the ε carbide has been dissolved. It was stated that stress intensity factor K_Ic significantly decreases in the case of the presence of dispersive elongated cementite precipitations at the boundaries of the prior austenite grains.     

Microstructure characterization in the weld metals of HQ130 + QJ63 high strength steels

Microstructural characterization of the weld metals of HQ130 + QJ63 high strength steels, welded under 80% Ar + 20% CO₂ gas shielded metal arc welding and different weld heat inputs, was carried out by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The relative contents of acicular ferrite (AF) and pro-eutectic ferrites (PF) in the weld metals were evaluated by means of XQF-2000 micro-image analyser. The experimental results indicate that there is acicular ferrite in the grain and some pro-eutectic ferrite on the boundary of original austenite grains when the weld heat input is small (E = 9.6 kJ/cm), but the main microstructure is ferrite side plate (FSP) when the heat input is larger (E = 22.3 kJ/cm). So the weld heat input should be strictly controlled in the range 10 ∼ 20 kJ/cm and then the content of pro-eutectic ferrite is limited to < 25%. Thus weld metals of HQ130 + QJ63 high strength steels with high toughness and excellent resistance to cracking can be ensured.     

Effect of laser incident energy on microstructures and mechanical properties of 12CrNi2Y alloy steel by direct laser deposition

This work aims to establish the effect of laser energy area density(EAD) as the laser incident energy on density, microstructures and mechanical properties of direct laser deposition(DLD) 12CrNi2 Y alloy steel.The results show that the density of DLD 12CrNi2 Y alloy steel increases at initial stage and then decreases with an increase of EAD, the highest density of alloy steel sample is 98.95%. The microstructures of DLD12CrNi2 Y alloy steel samples are composed of bainite, ferrite and carbide. With increase of EAD, the microstructures transform from polygonal ferrite(PF) to granular bainite(GB). The martensite-austenite constituent(M-A) in GB transforms from flake-like paralleling to the bainite ferrite laths to granular morphology. It is also found that the average width of laths in finer GB can be refined from 532 nm to 302 nm, which improves the comprehensive properties of DLD 12 CrNi2 Y alloy steel such as high hardness of 342 ± 9 HV_(0.2), yield strength of 702 ± 16 MPa, tensile strength of 901 ± 14 MPa and large elongation of15.2%±0.6%. The DLD 12CrNi2 Y material with good strength and toughness could meet the demand of alloy steel components manufacturing.     

Effects of cooling process on microstructure and hardness for 1·0C–1·5Cr bearing steel

Effects of cooling rate (V_cr) and final cooling temperature (T_ft), after hot deformation, on microstructure and hardness for 1·0C–1·5Cr bearing steel were investigated. The results show that if V_cr increases from 2 to 25°C s^?1 and T_ft remains at 650°C, pearlite colony size and grain size both decrease, hardness increases. When V_cr exceeds 8°C s^?1, carbide network can be restrained effectively. TEM micrographs indicate that there exist branches in the local region of lamellar cementite and ferrite, and a ferrite thin film is also found around the proeutectoid carbide. Under the cooling rate of 10°C s^?1, with the increase in T_ft, the microstructure changes from martensite into pearlite, carbide network becomes more serious and hardness decreases.     

Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

Abstract

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (C _max/C _min) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium k _Cr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.     

10CrNiCu钢激光-MAG复合焊热循环及组织特性研究

采用热电偶测温技术测量了激光-MAG复合焊及常规MAG焊接头粗晶区热循环曲线,并通过金相显微镜对比分析了两种焊接接头粗晶区组织特征。研究结果表明:与常规MAG焊相比,HLAW热循环加热及冷却速度较大,高温停留时间短,呈陡升陡降特征;粗晶区主要由粒状贝氏体、贝氏体、贝氏体基体上析出粒状碳化物组织及细小板条状马氏体组成;M-A岛含量较少,以细小颗粒状弥散分布在铁素体基体上。     

Effect of tempering temperature on the microstructure and properties of ultrahigh-strength stainless steel

The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M₇C₃ carbide precipitated when the steel was tempered at 400 °C, and M₇C₃ and M₂C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M₂C and M₂₃C₆ carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M₇C₃ carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness.     

Microstructure and properties of carbide free bainite railway wheels produced by programmed quenching

Abstract

A kind of carbide free bainite steel was used to produce railway wheels. The alloy design, manufacture process, microstructure and behaviours are summarised. The novel steel is distinctively superior in excellent combination of strength and toughness compared with the traditional pearlite type wheel steels. The details of the microstructure of novel steel have been investigated in nanoscale.