Low temperature mechanical properties of quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling

Abstract

The mechanical properties of a quenched and tempered 0·4C–Ni–Cr–Mo steel after controlled rolling (CRP steel) have been studied over the temperature range 77–293 K with the aim of developing a CRP steel for low temperature ultrahigh strength applications. The results obtained were compared with those of a conventional quenched and tempered 0·4C–Ni–Cr–Mo steel (CHT steel). The CRP process was found to improve greatly the strength, ductility, and fracture and impact toughness for tempers at and below 473 K, independent of test temperature, but there was some concomitant deterioration in the transverse properties. It is postulated that the fine subcell structure, introduced during the CRP, is mainly responsible for the improved mechanical properties. However, there is an abrupt reduction in fracture energy of fatigue precracked steels for tempers above 473 K, so above this temperature there is little difference in the properties of the CRP and CHT steels. This is attributed to fine carbide precipitation, which promotes shear localisation and dimple fracture. Despite this, it is demonstrated by the present work that the CRP steel is attractive for low temperature ultrahigh strength steel applications.

MST/734     

Microstructural features affecting fracture toughness of high strength steels

The fracture toughness of quenched and tempered steels, such as AISI 4340, AISI 4130 and 300M, can be increased by 50–100% by minor changes in heat treating procedures. Certain microstructural features, particularly blocky ferrite, upper bahnte and twinned martensite plates, are deleterious to fracture toughness. Similarly, the presence of undissolved carbides and sulfide inclusions, which act as crack nuclei, can lower fracture toughness by 25–50%. Other microstructural constituents, such as lower bainte, autotempered martensite, and retained austenite can enhance fracture toughness. By controlling the amounts and distributions of the microstructural constituents, the fracture toughness values of AISI 4340, AISI 4130 and 300M can be raised to the fracture toughness level of 18Ni maraging steel at equivalent values of yield strength.     

Shape effect of ultrafine-grained structure on static fracture toughness in low-alloy steel

Abstract

A 0.4C-2Si-1Cr-1Mo steel with an ultrafine elongated grain (UFEG) structure and an ultrafine equiaxed grain (UFG) structure was fabricated by multipass caliber rolling at 773 K and subsequent annealing at 973 K. A static three-point bending test was conducted at ambient temperature and at 77 K. The strength–toughness balance of the developed steels was markedly better than that of conventionally quenched and tempered steel with a martensitic structure. In particular, the static fracture toughness of the UFEG steel, having a yield strength of 1.86 GPa at ambient temperature, was improved by more than 40 times compared with conventional steel having a yield strength of 1.51 GPa. Furthermore, even at 77 K, the fracture toughness of the UFEG steel was about eight times higher than that of the conventional and UFG steels, despite the high strength of the UFEG steel (2.26 GPa). The UFG steel exhibited brittle fracture behavior at 77 K, as did the conventional steel, and no dimple structure was observed on the fracture surface. Therefore, it is difficult to improve the low-temperature toughness of the UFG steel by grain refinement only. The shape of crystal grains plays an important role in delamination toughening, as do their refinement and orientation.     

Recrystallization of molybdenum- and nitrogen- alloyed austenitic stainless steels after hot working

Abstract

Hot compression experiments have been performed to study the influence of molybdenum and nitrogen on the kinetics of static recrystallization in four austenitic stainless steels: AISI 316L (17Cr–13Ni–2·5Mo), AISI 316LN (17Cr–13Ni–2·5Mo–0·17N), DIN Wnr.l·4439(18Cr–12Ni–4·4Mo–0·2N), and UHB 904L(20Cr–25Ni–4·5Mo–1·5Cu). Experiments were carried out both for wrought and cast materials at temperatures of 1050–1250°C and to strains between ε = 0·10 and ε = 0·40. It was found that the static recrystallization was delayed by a factor of about 1·7 in time when raising the molybdenum content from 2·5 to 4·5 wt-%. Nitrogen was found to have no significant effect on the rate of static recrystallization. Cast material recrystallized more slowly than wrought material and this could not be attributed only to a grain size effect. Increasing strain and temperature resulted in a reduced recrystallization time. The recrystallized grain size decreased with increasing strain and decreasing temperature. Empirical expressions could reproduce with excellent accuracy the strain and temperature dependence of both the fraction recrystallized and the recrystallized grain size.

MST/360     

Effects of vanadium on upper-nose temper embrittlement and other Mechanical properties of Cr–Ni–Mo low-alloy steels

Abstract

Large components manufactured from Cr–Mo–Ni low-alloy steels are usually heavily tempered for the purpose of stress relieving, with resultant undesirable loss of strength and the inception of upper-nose temper embrittlement (UNTE). This paper describes an investigation on the effects of vanadium additions and of variation in the molybdenum content on the properties of these heavily tempered steels. It is shown that the addition of vanadium to these steels leads to a substantial improvement in their strength without impairment of their ductility and toughness, and also to a marked improvement in their resistance to UNTE. An increase in the molybdenum content of the steel from 0·5 to 1·0% leads to a moderate improvement in its strength, but has an aggravating effect on its susceptibility to UNTE, as a result of an increase in the coarsening rate of the grain boundary carbides and the formation of unfavourable M₂₃C₆ and M_aC_b molybdenum-rich large carbides.

MST/390     

Upper shelf temperature dependence of fracture toughness for four low to intermediate strength ferritic steels

To examine the inadequacy and expose the potential consequences of utilizing transition temperature range fracture toughness values when performing fracture mechanics analyses on structures operating at upper shelf temperatures, the upper shelf fracture toughness behavior of three rotor forging steels (ASTM A471 NiCrMoV, ASTM A470 CrMoV and AISI 403 modified 12 Cr) plus an ASTM A217 $\text{2}\text{1}\text{4}\text{Cr}$-lMo cast steel was investigated. Elastic-plastic fracture toughness values were obtained via the $\text{J}{}_{\mathrm{IC}}$ resistance curve test technique. Except for the CrMoV rotor steel, upper shelf fracture toughness peaked near the temperature where zero percent brittle fracture first occurred in the Charpy V-notch impact tests after which decreases of 25–45% in toughness were realized. Fracture toughness tests were conducted up to 800°F.     

Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

Influence of heat treatments on toughness and sensitization of a Ti-alloyed supermartensitic stainless steel

Supermartensitic steels are a new class of martensitic stainless steels developed to obtain higher corrosion resistance and better toughness through the reduction of carbon content, and addition of Ni and Mo. They were developed to more critical applications or to improve the performance obtained with conventional grades AISI 410, 420, and 431. In this study, the influences of the tempering parameters on the microstructure, mechanical properties (hardness and toughness), and sensitization of a Ti-alloyed supermartensitc stainless steel were investigated. The material showed temper embrittlement in the 400–600 °C range, as detected by low temperature (−46 °C) impact tests. The degree of sensitization measured by double loop reactivation potentiodynamic tests increased continuously with the increase of tempering temperature above 400 °C. Healing due to Cr diffusion at high tempering temperatures was not observed. Double tempered specimens showed high amounts (>20%) of reverse austenite but their toughness were similar to specimens single tempered at 625 and 650 °C.     

Determination of dynamic fracture toughness of heat affected zone of 9Cr–1Mo steel from instrumented drop weight tests

Abstract

The dynamic fracture toughness (K_1d) of the heat affected zone (HAZ) of 9Cr–1Mo steel at and below the nil ductility transition temperature has been estimated from instrumented drop weight test results. The presence of a significant microstructural gradient in the HAZ, comprising coarse, fine, and intercritical regions with sharp toughness differences, is reflected in the presence of distinctive load peaks in the load–time traces; the K_1d estimates for the coarse, fine, and intercritical regions are 53, 85, and 128 MPa m^1/2 respectively. The results from the 9Cr–1Mo steel were compared with those for AISI grade 403 martensitic stainless steel. The lack of distinctive multiple load peaks in the load–time traces for the latter is attributed to the absence of a steep toughness gradient, owing to a more or less uniform martensitic microstructure in the HAZ.     

Analyses of the failures on shear cutting blades after trimming of ultra high-strength steel

Damages on shear cutting blades were analyzed after 50,000 strokes of trimming on an ultra high-strength steel sheet. Traditional D2 alloy and an advanced one (Cr08H) based on the composition of 1C-8Cr were quenched from 1030 °C, tempered at 180 °C and submitted to the shear cutting test. Cr08H had lower hardness, a smaller volume fraction of M₇C₃ carbides while it contained a larger volume fraction of retained austenite. And these resulted in more scratches and rounded edges because of degraded resistance to wear and local plastic deformation. In spite of higher impact toughness, Cr08H exhibited inferior resistance to chipping which was the consequence of localized brittle fracture. It could be concluded that this was caused by more transformation of austenite as well as by insufficiently hardened matrix, both of which were attributed to inappropriate conditions of the heat treatment.     

Development of carbon—Low alloy steel grades for low temperature applications

Low alloy steels are processed to fulfill the requirements of low temperature applications. Besides the chemical composition, the steel should receive a suitable heat treatment to ensure the targeted mechanical properties at low temperature. In other words, the steels are designed to delay the ductile to brittle transition temperature to resist dynamic loading at subzero temperatures. Steel alloys processed for liquefied gas pipeline fittings are examples for applications that need deep subzero impact transition temperature (ITT).The main purpose of the present work was to find a suitable heat treatment sequence for alloys LC2 and LC2-1. Further, it aimed to correlate the impact toughness with the microstructure and the fracture surface at different sub-zero temperatures.The steels under investigation are carbon-low alloy grades alloyed with Ni, Cr and Mo. LC2 steel alloy has been successfully processed and then modified to LC2-1 alloy by addition of Cr and Mo. Oil quenching from 900 °C followed by tempering at 595 °C was used for toughness improvements. Hardness, tensile and impact tests at room temperature have been carried out. Further impact tests at subzero temperatures were conducted to characterize alloys behavior. Metallographic as well as SEM fractographic coupled with XRD qualitative analysis are also carried out.Non-homogenous martensite-ferrite cast structure in LC2 was altered to homogeneous tempered martensite structure using quenching-tempering treatment, which is leading to shift the ITT down to −73 °C. Addition of Cr and Mo creates a very fine martensitic structure in LC2-1 alloy. Quenching-tempering of LC2-1 accelerates ITT to −30 °C. It is expected that the steel was subjected to temper embrittlement as a result of phosphorus segregation on the grain boundary due to Cr and Mo alloying, as it was concluded in reference no. [6].     

Combined quenching and tempering induced phosphorus segregation to grain boundaries in 2·25Cr–1 Mo steel

Abstract

Combined quenching and tempering induced phosphorus segregation to prior austenite grain boundaries in α 0·077 wt-%P doped 2·25Cr–1Mo steel was examined using field emission gun scanning transmission electron microscopy. The results indicate that combined equilibrium and non-equilibrium phosphorus segregation may play an important part in temper embrittlement of the steel caused by direct tempering after quenching. Non-equilibrium segregation requires the formation of sufficient quantities of vacancy–impurity complexes and their migration to grain boundaries is of great importance in the segregation. For this reason, the mechanism for migration of the complexes is discussed in detail.

MST/3419     

Laser surface hardening of AISI 01 tool steel and its microstructure

Abstract

The effects of laser surface hardening on AISI 01 tool steel samples were studied by changing the laser operating parameter combinations and the initial steel microstructure. Both melted and solid state transformed regions were produced, and then studied using optical microscopy, analytical electron microscopy, X-ray diffraction, and measurements of micro hardness to investigate the hardening mechanisms and the development of compressive residual stresses. The results indicate that hardened case depths up to 0·6 mm can be obtained using a laser beam operated at a power of 500 W and a scan rate of 2·1 mm s^?1, but that different amounts of retained austenite and undissolved carbides are observed for different beam powers. Quenched and tempered AISI 01 steel samples, with initial hardness values in the range 30–40 HRC, are better suited for laser surface hardening compared with the samples with initial hardness of 48–50 HRC, because the formation of an over tempered region adjacent to the hardened zone can be avoided.

MST/901     

An evaluation of the crack growth and fracture properties of AISI 403 modified 12 Cr stainless steel

The plane strain fracture toughness, $\text{K}{}_{\mathrm{Ic}}$, and fatigue crack growth rate material properties were developed for three heats of AISI 403 modified 12 Cr stainless steel. Valid (per ASTM requirements) fracture toughness tests were conducted in the temperature range ?200°F to 175°F. In addition, both the room temperature air environment plus 520°F, 1200psi distilled water environment fatigue crack growth rate material properties are presented. Finally, a hypothetical example problem is included which demonstrates the application of fracture mechanics technology to an AISI 403 modified 12 Cr stainless steel turbine rotor.     

Tem observation and fracture morphology in the cghaz of a new 0cr18mo2ti ferritic stainless steel

Microstructure, precipitates and fracture morphology in the coarse grained heat-affected zone CGHAZ) of a new high-purity 0Cr18Mo2Ti ferritic stainless steel were studied by means of optical metallography, SEM, TEM, X-ray diffractometer, etc. Experimental results indicated that grain coarsening resulted in brittle fracture in the CGHAZ of 0Cr18Mo2Ti steel. The reduction of impact toughness in the CGHAZ due to change of cooling rate can be attributed to the increase of nitrides (TiN, Cr₂N, etc). These nitrides in the CGHAZ promote initiation and propagation of brittle cracks. The precipitated Cr₂N nitrides in the grain boundaries decrease impact toughness in the CGHAZ of 0Cr₁₈Mo₂Ti steel by promoting crack initiation. In practical applications, the welding heat input (E) should be as low as possible to prevent toughness reduction in the CGHAZ.     

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb-Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb-Cu-B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.     

Formation of carbide layers on AISI H13 and D2 steels by treatment in molten borax containing dissolved both Fe–Nb and Fe–Ti powders

In this work, AISI H13 and D2 tool steels were treated in molten borax, containing dissolved ferro-niobium, ferro-titanium and aluminum, at 1020 °C for 4 h. Samples were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD) and Vickers microhardness. Well defined layers were obtained with excellent thickness regularity. For the AISI H13 steel the layer measured 9 μm and for the AISI D2 steel the layer thickness was 18 μm. Their microhardness values were at about 2600 HV_0.050. The layers consisted of niobium carbide according to XRD analysis. EDS results showed the predominance of niobium and absence of iron in the layers on both steels. The presence of titanium was detected, just in small amounts, in the region of the layer next to substrate.     

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.     

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.     

The effect of tempering temperature on mechanical properties and microstructure of low alloy Cr and CrMo steel

Two low alloy Cr and CrMo steels with similar levels of carbon, manganese and chromium have been studied to determine the effect of tempering temperature on the mechanical properties and microstructure. The quenching and tempering of steels were carried out using a high-speed dilatometer. The steels were quenched at the average cooling rate of 30 K s-1 in the temperature range from 1123 to 573 K by flowing argon and tempered at 673, 823 and 973 K. The martensite of steels formed during quenching was of entire lath morphology with 2 vol% retained austenite. It was found that after tempering at 973 K the Cr steel contained only orthorhombic cementite, while the CrMo steel contained the cementite and hexagonal Mo2C particles in the ferrite matrix. At the same tempering conditions, the CrMo steel shows higher strength but lower ductility as compared to those of Cr steel. It is shown that this difference results from finer prior austenite grain, substructure within matrix and precipitate dispersion strengthening, primarily by Mo2C. Transmission electron microscopy (TEM) bright- and dark-field micrographs as well as selected area diffraction pattern analysis of orientation relationship showed that the cementite precipitated from the ferrite matrix. Fractography analysis showed that the morphology fracture surface was changed by increasing tempering temperature. Tempering at 973 K obtained ductile fracture by the microvoid coalescence mechanism.