Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.     

Serrated yielding in 9Cr–1Mo ferritic steel

Abstract

Tensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300–873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr–1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load–elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol^-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr–1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations indicate reduced propensity to DSA as a result of increased precipitate sinks as well as a reduced carbon concentration in solid solution owing to an increased density of carbides in the thermally aged conditions. Reduced propensity to DSA resulted in a significant reduction in the strength values at intermediate temperatures.     

Prediction of the temperature dependence of fracture toughness of 12Cr–2Ni–Mo refractory steel

We study the influence of temperature and the size of the specimens on the characteristics of static crack resistance of 12Cr–2Ni–Mo refractory steel. It is shown that, in the temperature range 20–450°C, the increase in the thickness of specimens leads to an insignificant increase in fracture toughness obtained along a 5% secant line according to the standards of evaluation of the characteristics of crack resistance. The evaluation of the characteristics of crack resistance of 12Cr–2Ni–Mo steel with regard for the scale effect according to an earlier developed numerical-experimental model reveals the existence of satisfactory agreement with the experimental data in the entire investigated temperature range. Translated from Problemy Prochnosti, No. 4, pp. 78–88, July–August, 2009.     

Cleavage fracture in 26Cr–1Mo ferritic stainless steel

Abstract

Cleavage fracture of a 26Cr–1Mo ferritic stainless steel has been studied using fatigue precracked specimens. The parameters determined were fracture toughness, cleavage fracture strength, and effective surface energy of ferrite. The results have been compared with earlier results on notched specimens.

MST/185     

Hydrogen and temper embrittlement in 9Cr–1Mo steel

Abstract

The synergism between hydrogen embrittlement and temper embrittlement has been investigated in a 9Cr–1Mo martensitic steel. Measurements of tensile ductility were used to monitor the development of embrittlement with increasing hydrogen content in material as tempered and aged for up to 5000 h at 500 or 550°C. A detailed examination was made of associated changes in fracture mechanism, precipitate microstructure, and interfacial and precipitate chemistry. A strong interaction between hydrogen and temper embrittlement was observed. Both types of embrittlement in isolation reduced tensile ductility by promoting a ductile interlath fracture mechanism: ‘chisel fracture’. Hydrogen and temper embrittlement acted synergistically to reduce ductility further by the promotion of brittle intergranular fracture and transgranular cleavage. The dominant factor controlling the interaction was the precipitation of a brittle intermetallic Laves phase containing phosphorus in solution. Phosphorus segregated to interfaces was considered to make an important, but secondary, contribution to the embrittlement observed.

MST/791     

Cooling effects on microstructure and mechanical properties of 27Cr–4Mo–2Ni ferritic stainless steel

The effects of cooling manner on the microstructure and mechanical properties of 27Cr–4Mo–2Ni ferritic stainless steel were investigated. It was found that the Laves phase (except for the TiN and Nb(C, N) particles) was distributed both in the grains and at the grain boundaries in the furnace-cooled specimen. The water-quenched and air-cooled specimens showed only TiN and Nb(C, N) particles. After annealing at 1100°C, the furnace-cooled specimen showed significant grain coarsening as compared to the water-quenched and air-cooled specimens. Furthermore, the Vickers hardness of the furnace-cooled specimen increased, while the total elongation decreased because of the formation of the Laves phase. The precipitation of the Laves phase resulted in the brittle fracture of the specimen during the tensile test.     

Morphological changes and hardness evolution in Cu–30Ni–5Cr and Cu–45Ni–15Cr spinodal alloys

Abstract

The development of increased strength in Cu–Ni–Cr alloys, compared with binary Cu–Ni alloys, is dependent upon heat treatment. These alloys have compositions which permit them to be solution treated at elevated temperature and then aged at a lower temperature, in a two phase field, to produce hardening. Decomposition into two phases may occur by nucleation and growth or by a spinodal reaction, depending on alloy composition and heat treatment temperature. As part of a more extensive study of ternary Cu–Ni–Cr alloys, the decomposition of Cu–30Ni–5Cr and Cu–45Ni–15Cr (wt-%) has been studied in the spinodal range. The evolution of microstructure has been determined together with the coarsening kinetics for the modulated spinodal decomposition products. Specimens rapid quenched from 1050°C, were aged in the temperature range 300–800°C. The progress of spinodal decomposition was followed via hardness measurements, X-ray diffraction, and scanning and transmission electron microscopy. Modulation wavelengths were measured from both X-ray diffraction patterns and electron micrographs. It was found that during the early stages of aging the modulation wavelength remained constant while the hardness increased continuously. After a certain period of aging, the hardness remained constant at its peak value, while the modulation wavelength increased continuously. The results are consistent with current theories of spinodal decomposition and hardening.

MST/1733     

Methods to overcome embrittlement problem in 9Cr–1Mo ferritic steel and its weldment

This article presents the issues that need to be addressed in ferritic steel, for their use in nuclear core, namely, the embrittlement and type IV cracking of weldment. It has been established that the ferritic steels possess a significantly higher resistance to radiation damage as compared to the present generation austenitic stainless steels and the creep behavior is satisfactory for applications up to 873 K. The major challenges that need to be addressed are the poor creep resistance of the weld joints and embrittlement of ferritic steels. This article describes the efforts taken at IGCAR to overcome the embrittlement problem by impurity control, grain boundary engineering or design of suitable thermomechanical treatments in a 9Cr–1Mo ferritic steel.     

Surface tension and density data for Fe–Cr–Mo,Fe–Cr–Ni,and Fe–Cr–Mn–Ni steels

The temperature dependence of surface tension and density for Fe–Cr–Mo (AISI 4142), Fe–Cr–Ni (AISI 304), and Fe–Cr–Mn–Ni TRIP/TWIP high-manganese (16 wt% Cr, 7 wt% Mn, and 3–9 wt% Ni) liquid alloys are investigated using the conventional maximum bubble pressure (MBP) and sessile drop (SD) methods. In addition, the surface tension of liquid steel is measured using the oscillating droplet method on electromagnetically levitated (EML) liquid droplets at the German Aerospace Centre (DLR, Cologne). The data of thermophysical properties for Fe–Cr–Mn–Ni is of major importance for modeling of infiltration and gas atomization processes in the prototyping of a “TRIP-Matrix-Composite.” The surface tension of TRIP/TWIP steel increased with an increase in temperature in MBP as well as in SD measurement. The manganese evaporation with the conventional measurement methods is not significantly high within the experiments (?_Mn < 0.5 %). The temperature coefficient of surface tension (dσ/dT) is positive for liquid steel samples, which can be explained by the concentration of surface active elements. A slight influence of nickel on the surface tension of Fe–Cr–Mn–Ni steel was experimentally observed where σ is decreased with increasing nickel content. EML measurement of high-manganese steel, however, is limited to the undercooling state of the liquid steel. The manganese evaporation strongly increased in excess of the liquidus temperature in levitation measurements and a mass loss of droplet of 5 % was observed.     

Magnetic nondestructive evaluation of thermally degraded 2.25Cr–1Mo steel

Aging was performed to understand the microstructural degradation in 2.25Cr–1Mo steel. Microstructural parameters (mean equivalent carbide size, number of carbides per unit area), mechanical properties (UTS, Vickers hardness) and magnetic properties (coercivity, remanence) were measured to investigate the relationship among these parameters. The magnetic coercivity and remanence were observed to decrease rapidly in the initial 1000 h of aging time and then decrease slowly thereafter. Linear correlations between mechanical and magnetic properties were found.     

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     

Hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC) of 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener

This paper brings a failure case study of high strength 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener, which failed due to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). 0.35C–3.5Ni–1.5Cr–0.5Mo steel in hardened and tempered condition, meeting the specified axial tensile stress rating of 1250 MPa is widely used as fasteners in space programmes.In the course of assembly of the structural parts of a satellite launch vehicle, 10 nos of fasteners developed cracks on tightening using a torque wrench set to 6 N m torque surprisingly.Also some fasteners, which were under assembly load of more than 6 months in the same vehicle assembly, were found to be cracked.The failure was attributed to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). The details of the analysis and mechanism involved in the HI-IGSCC are presented in detail.Detailed metallurgical analyses of the cracked fasteners support the successive steps of the corrosion enhanced plasticity model, which is based on a local softening in the SCC crack region. The mechanism of a dislocation pileup ahead of a crack under corrosion and stress due to diffusing hydrogen promotes stress concentration against micro-obstacle and caused failure.     

Effects of Cr Content on the Microstructure and Properties of 26Cr–3.5Mo–2Ni and 29Cr–3.5Mo–2Ni Super Ferritic Stainless Steels

By using scanning electron microscopy,energy-dispersive spectrometry,X-ray diffraction,strength and hardness measurements,the microstructure,precipitation,mechanical properties,and corrosion resistance have been investigated for two super ferritic stainless steels,26Cr–3.5Mo–2Ni and 29Cr–3.5Mo–2Ni,with the aim to consider the effect of Cr content.The results showed that with the addition of Cr content,the recrystallization temperature was increased;the precipitation of Laves and Sigma(σ)phases was promoted and the mechanical properties of super ferritic stainless steel were modified.Furthermore,the pitting corrosion resistance and corrosion resistance to H_2SO_4 of the two super ferritic stainless steels were improved.In addition,suitable annealing processing is a key factor to maintain integrated performance by optimizing microstructure and removing detrimental precipitation phases.     

Identification of processing parameters for Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel using processing maps

Abstract

The processing parameters for hot working of Fe–15Cr–2.2Mo–15Ni–0.3Ti austenitic stainless steel (alloy D9) are identified using processing maps developed on the basis of the dynamic materials model and hot compression data in the temperature range 850–1250°C and strain rate range 0.001–100 s^-1. The efficiency of power dissipation increased with increase in temperature and decrease in strain rate. Dynamically recrystallised microstructures resulted when the efficiency of power dissipation was in the range 27–37%, i.e. in the temperature range 1050–1250°C and strain rate range 0.001–0.5 s^-1. Flow localisation occurred in the regions of instability at temperatures lower than 1000°C and at higher strain rates. The dynamic recrystallisation regime observed in this alloy is compared with other austenitic stainless steels, namely, AISI type 304L and 316L.     

Adhesion characteristics of Cu/Ni–Cr/polyimide flexible copper clad laminates according to Ni:Cr ratio and Cu electroplating layer thickness

The adhesion strength of Cu/Ni–Cr/polyimide flexible copper clad laminate (FCCL) was evaluated according to the composition ratio of the Ni–Cr layer and the thickness of the Cu electroplating layer, by using a 90° peel test. The changes in the morphology, chemical bond and adhesion property were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The peel strength of the FCCL increased with increasing Cr content and increasing Cu electroplating layer thickness. This increasing FCCL peel strength was attributed to a lower C–N bond and higher C–O and carbonyl (C=O) bonds in the polyimide surface compared to the FCCL with a lower adhesion strength. The adhesion property of the FCCLs was significantly affected by the Ni:Cr ratio.     

Effect of frequency on very high cycle fatigue behavior of a low strength Cr–Ni–Mo–V steel welded joint

Axially fully-reversed fatigue test of a low strength Cr–Ni–Mo–V steel welded joint was conducted up to the very high cycle fatigue regime under the frequency of 110 Hz and 20 kHz. The S–N curve shows a duplex shape at low frequency while decreases continuously at high frequency. Sites of crack initiation and fracture of the welds depend on stress level and loading frequency, hence leading to changed fatigue strength. In addition, frequency effect varies among different parts of the welded joint and fatigue lifetime.     

Oxide/metal interface on 20Cr–25Ni–Nb stainless steel

Abstract

20Cr–25Ni–Nb stabilised stainless steel is used to contain the fuel in the advanced gas cooled reactor. During operation, this steel must withstand temperatures from 600 to 1073 K in CO₂ gas at 40 atm pressure. It is important that the oxide which forms on this steel is thoroughly characterised and the adherence of the oxide to the metal is understood. A technique of sputter ion plating has been used to remove the oxide from the metal without destroying either metal or oxide. This involves plating the oxide with nickel or molybdenum at a temperature of 600 K, while sputtering the surface with argon ions. On cooling, stresses set up between the oxide and the metal cause the oxide plus sputtered layer to peel off allowing both the metal and oxide sides of the interface to be examined. Results are presented from studies of the metal/oxide interface using scanning Auger microscopy. Analysis of grain centres and grain boundaries indicates that silicon and chromium play an important role in oxide/metal adhesion and, together with conventional analysis of the bulk oxide, assist in determining the oxidation mechanism.

MST/862     

Ni–Cu alloy for diffusion bonding cermet/steel in air

Microstructural and mechanical evaluation for joints obtained by static and dynamic diffusion bonding of a 90MnCrV8 high strength steel coated with WC–Co, using a Ni–Cu alloy as interlayer, are shown in the present work. In all joints different reacted zones generated during the bonding process can be distinguished by means of scanning electron microscopy and dispersive X-ray spectrometry. The maximum tensile strength obtained using dynamic diffusion bonding process confirms a very promising technology for industrial applications.     

Zone-wise investigation of creep behaviour 9Cr–1Mo steel weld joints

Distinct regions such as weld metal, heat-affected zone (HAZ) and base metal of P9 steel weld joints fabricated by various welding processes were investigated using impression creep testing. Smaller prior austenitic grain size, lower density of precipitates and dislocations resulted in faster recovery and higher creep rate of HAZ in comparison to the weld and base metal. Compared to base metal, shielded metal arc weld (SMAW) and activated tungsten inert gas (A-TIG) weld of the P9 steel weld joints exhibited better resistance to creep and displayed higher activation energy due to their coarser prior austenite grain size. A-TIG HAZ exhibited superior creep properties compared to the SMAW and TIG HAZ due to the presence of higher number density of precipitates.     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.