Double Tempering Research Articles

Microstructure, heat treatment and pitting corrosion of 13CrNiMo plate and weld metals

Potentiodynamic measurements together with SEM were used to analyze susceptibility to pitting corrosion of 13CrNiMo plate metals. Volume fractions of austenite in tempered conditions affect pitting resistance. Characteristic potentials (critical pitting and repassivation) increase according to the retained austenite content. Results correspond to a structural refinement resulting from a double tempering procedure. A comparison is made with results from weld metal samples. The importance of this evaluation is to be able to determine pitting resistance, either in a service application, or in laboratory test programs.

Influence of austenitizing temperature on microstructure and mechanical properties of AISI 431 martensitic stainless steel electron beam welds

The relative effects of various austenitizing temperatures on microstructure and mechanical properties of electron beam welds of AISI 431martensitic stainless steel were studied. The post-weld heat treatments consist of austenitizing the weld samples for 1 h at various temperatures, i.e., at 950 °C, 1000 °C, 1050 °C, 1100 °C and at 1150 °C and air cooling followed by double tempering, i.e., tempering at 670 + 600 °C. In the as-welded condition the microstructure contains dendritic structure with ferrite network and retained austenite in a matrix of un-tempered martensite. The prior austenite grain size increased with increase in austenitizing temperature. Parent metal grain size was coarser as compared to grain size in the weld zone in respective conditions. Retained austenite content increased with increase in the austenitizing temperature. Presence of undissolved carbides was observed in welds and parent metal austenitized up to 1000 °C and they dissolved at austenitizing temperature ⩾ 1050 °C. Coarsening of martensite laths was observed after tempering. The martensite laths were coarser in the samples subjected to higher austenitizing temperatures. Optimum mechanical properties, i.e., strength, hardness and toughness were observed when austenitized between 1050 °C and 1100 °C followed by tempering. Austenitizing at 1150 °C and tempering resulted in inferior mechanical properties. The mechanism for the observed trends is discussed in relation to the microstructure, fracture features and mechanical properties.

Influence of post-weld heat treatments on microstructure and mechanical properties of AISI 431 martensitic stainless steel friction welds

The relative effects of different post-weld heat treatments (PWHTs) on microstructure and mechanical properties of friction welded martensitic stainless steel type AISI 431 were studied. The weld microstructure consists of acicular martensite in equiaxed prior austenite grains and with heat treatment, the martensitic microstructure experiences coarsening. It was observed that retained austenite content decreases with an increase in the PWHT tempering temperature. The tensile properties of welds under different PWHTs were comparable to those of parent metal in the respective treatments. Welds exhibited higher notch tensile strengths than the parent metal in the respective heat treatment condition. Double tempering (670°C + 600°C) led to maximum reduction in hardness. Welds exhibited poor impact toughness in the as welded condition. The improvement in impact toughness was found satisfactory in welds subjected to double tempering, although did not match to the level of the parent metal at respective conditions. The mechanism and reasons for the observed behaviour have been discussed, correlating the microstructure, fracture features and mechanical properties.

Application of Fractional Design of Experiments to the Optimisation of the Hardness and Microstructure of Duplex Cast Rolls

AbstractIn the present paper, the results of a fractional design of experiments (DOE) with 16 experiments for 7 factors and resolution IV are presented. The goal was to identify the active roll manufacturing factors that influence roll barrel hardness after a double tempering treatment, the primary austenite grain size and its volume fraction, and the volume fraction of ledeburite in the outer ring layer of duplex work rolls made of Nihard cast iron. As a primary conclusion, both the total Si percentage and the amount of SiCaMn inoculant added to the ladle were found to have a significant effect on the volume fraction of ledeburite. The statistical analysis also showed that high Si percentages and heavier ring roll layer weights were the significant factors promoting the increase in the size of the gamma grains. High Si contents present in the melt and the addition of SiCaMn as inoculant seem to have a counter effect whereby the volume fraction of stable eutectic gamma increases and the volume fraction of proeutectic gamma decreases, with a positive balance for the sum of these austenites of different origins. Finally, the factors that have an influence on roll barrel hardness are those factors that promote the chill effect, such as low Si content, small amounts of liquid being poured into the mould to build up the outer ring layer, and the use of moulds without insulating sleeves.

Influencia de diversos tratamientos térmicos en la microestructura de una fundicion blanca con alto contenido en cromo

The aim of this research work was the heat treatment optimization of a high chrome white cast iron in order to obtain a final product with a high hardness (wear resistance). The effect of modifying the maintenance temperature during the different phases of the heat treatment (simple and double temperings and quenching followed by one or two temperings) on the microstructure and hardness of this product was evaluated. The necessity of performing the whole heat treatment, a quenching and a double tempering, was demonstrated in order to obtain the optimal properties of the product.

Corrosion studies of different ferrous alloys for rolling cylinders

The aim of this work is to study the corrosion behaviour in chloride media of a high chromium iron alloy. The influence of the surface finishing as well as the heat treatment was analysed. The technique employed for measuring the corrosion behaviour was the Electrochemical Impedance Spectroscopy, an electrochemical technique that allows the observation not only of the corrosion rates but also the changes on the corrosion mechanism of the alloy. The results showed that it is worth performing a double tempering and a having smoother surface finishing only for long exposure periods. A comparison between different oxidising conditions was also performed simulating the life in service of the hot rolling cylinders and revealed an increase in the protective properties as the oxidising time increased due to the chromium present on the alloy (18%).

A comparative study of the wear behaviour of sintered and laser surface melted AISI M42 high speed steel diluted with iron

Powders of AISI M42 high-speed steel (HSS) were blended with different proportions of water-atomised iron powders. The powders were subsequently submitted to uniaxial pressing and then divided in three lots. The first was submitted to sintering, the second was submitted to sintering plus laser surface melting (LSM) and the third was submitted to sintering plus LSM plus double tempering at the secondary hardening peak temperature of M42 HSS. The objective of this procedure was to evaluate the processing route that leads to reduced porosity in AISI M42 HSS and to higher abrasive wear resistance. Therefore the samples, with different chemical compositions and microstructures, were submitted to a detailed microstructural characterisation followed by microscale hardness and abrasive wear tests. It was observed that LSM leads to almost complete elimination of residual porosity and to the dissolution of large brittle carbides that are present in the as-sintered samples, leading to a homogeneous and extremely fine microstructure. This microstructure is formed of saturated plate martensite and a small proportion of retained austenite. The double tempering treatment, carried out in the laser surface melted samples samples, leads to the elimination of retained austenite and to a decrease of the lattice parameters of martensite due to the precipitation of thin carbides within martensite. As a result, while the hardness of the material in the sintered condition is between 245 and 625 HV (depending on the proportion of dilution with iron), after LSM the hardness is higher than 820 HV in all the samples. Surprisingly, the abrasive wear resistance of the laser melted and of the laser melted and tempered samples is lower than that of the as-sintered ones. Observation of the wear craters by scanning electron microscopy shows that this result is due to the different wear mechanisms acting on the samples processed by different routes.

Failure of a martensitic stainless steel pipe weld in a fossil fuel power plant

A main steam pipe of a newly commissioned boiler ruptured within a few hours of being put into service. Subsequent inspection revealed root cracking of several other main steam pipe joints. The failure investigation covered every aspect of welding, heating techniques and NDT inspection methods. X-20 is a martensitic grade of stainless steel having ∼0.2% carbon, which makes it highly prone to cracking in hardened condition. X-20 weld joints are given complex cycles of pre-heating and post-weld heat treatment to avoid cracking and to ensure that correct mechanical properties are achieved. The resistance-heating method was found to be inadequate for post-weld heat treatment of X-20 joints. As a result weld joints contained very hard and brittle martensite which facilitated propagation of cracks. The use of induction heating instead of resistance heating ensures the mechanical properties are within specified limits. It was demonstrated that double tempering heat treatment would give acceptable mechanical properties. Micro-cracks and fissures in the welds were identified as hydrogen induced cracking. Lowering the soaking temperature below 100°C in order to avoid double tempering is not advisable since it increases the risk of hydrogen induced cracking. It was concluded from failure investigation that every aspect of welding requires strict adherence to proven practices and there is no room for any experimentation. Electrode baking, pre-heating, post-weld heat treatment and ultrasonic inspection are critically important. Remedial actions were taken to ensure that the remaining joints were safe.

Toughness and Microstructure of 13Cr4NiMo High-Strength Steel Welds

The microstructures and tensile, Charpy, and crack tip opening displacement (CTOD) properties of 13Cr4NiMo soft martensitic stainless steel flux cored are welding process (FCAW) weld metals have been studied through different applied postweld heat treatments (PWHT). Phases and microstructural characteristics have been analyzed by scanning electron microscopy (SEM) and x-ray diffraction. The effect of the tempering and double tempering, with and without previous solution annealing, on the impact and fracture toughness has been studied. The role of the retained austenite resulting from tempering has been recognized, and it is suggested that the austenite particles improve the toughness of the welds through their transformation by the transformation-induced plasticity (TRIP) mechanism.

Effect of Single and Double Austenitization Treatments on the Microstructure and Mechanical Properties of 16Cr-2Ni Steel

Double austenitization (DA) treatment is found to yield the best combination of strength and toughness in both low-temperature as well as high-temperature tempered conditions as compared to single austenitization (SA) treatments. Obtaining the advantages of double austenitization (DA) to permit dissolution of alloy carbides without significant grain coarsening was attempted in AISI 431 type martensitic stainless steel. Structure-property correlation after low-temperature tempering (200 °C) as well as high-temperature double tempering (650+600 °C) was carried out for three austenitization treatments through SA at 1000 °C, SA at 1070 °C, and DA at 1070+1000 °C. While the increase in strength after DA treatment and low-temperature tempering at 200 °C is due to the increased amount of carbon in solution as a result of dissolution of alloy carbides during first austenitization, the increased toughness is attributable to the increased quantity of retained austenite. After double tempering (650+600 °C), strength and toughness are mainly found to depend on the precipitation and distribution of carbides in the microstructure and the grain size effect.

Use of carbonitriding for self-sharpening of tools in service

1. Carbonitriding at 800°C and heat treatment of flat blades of supershears and guillotines from steel 40Kh with subsequent grinding of the faces to a specific depth provides self-sharpening of the blades in operation. 2. In order to provide a self-sharpening regime after the creation of the diffusion layer on the faces of a blade and a hardening heat treatment, a part of the layer of the “soft” face is ground off to a hardness much lower than that of the surface of the “hard” face. 3. The structural strength of the blades is provided by optimum regimes of carbonitriding, quenching; and double tempering and raising the toughness of the “soft” face by a partial removal of the surface “hard” layer.

Characterization of Fatigue and Fracture Behaviour and Improvement of Toughness of End Fitting Material of Pressurized Heavy Water Reactors

An attempt has been made to characterise fracture and high cycle fatigue behaviour and to study the effect of microstructure on fatigue crack growth (FCG) and fracture toughness of AISI 403 material at room temperature. Fracture toughness evaluation of as received end fitting material in J-integral format showed large scatter which was thickness dependent. A considerable improvement in toughness, compared to as received material, resulted by imparting a double quench and tempering treatment (DQT). The toughness of DQT material was thickness independent and showed no scatter. The scatter in room temperature toughness value of as received material results from the fact that ductile to brittle transition temperature is in the vicinity of room temperature. The toughness improvement by DQT treatment appears to result from refinement of microstructure as well as from a change in carbide morphology compared to that of as received material. While thickness and microstructure had no influence on stage II fatigue crack growth (FCG) behaviour, the stage I FCG has been found sensitive to microstructure change.

Heat Treatment Response of Sintered M3/2 High Speed Steel Composites Containing Additions of Manganese Sulphide, Niobium Carbide, and Titanium Carbide

Abstract Metal matrix composites based on a sintered M3/2 high speed steel (HSS) and containing either single additions of TiC, NbC, or MnS particles or combined additions of MnS + TiC or MnS + NbC were sintered to full density and subjected to a conventional hardening heat treatment cycle of austenitisation, quenching, and double tempering. Relatively poor hardnesses and strengths were obtained in any composites that contained MnS particles, principally because of the weakening effects of MnS particles in the structure but also as a result of decarburisation of the steel matrix when oxygen introduced by the MnS particles reacted with carbon. Composites that contained TiC were changed in structure so that matrix M6C carbides were replaced by the less soluble MC carbide. They consequently showed slightly reduced hardening response compared to the baseline M3/2 HSS after heat treatment. Ball milling was necessary in powder mixtures that contained NbC to break up particle agglomerates. The resultant fine gra...

Roothaan–Hartree–Fock wave functions for cations and anions in Slater-type basis sets with doubly even tempered exponents

Roothaan-Hartree-Fock wave functions in Slater-type basis sets are reported for the cations Li+-Cs+ and anions H−-I− using the double even tempering (DET) method of selecting orbital exponents. The DET total energies do not differ from the corresponding numerical Hartree-Fock values by more than 0.2 millihartrees for the cations and anions. The present results together with the previous ones for neutral atoms [Theor Chim Acta 88:273 (1994)] provide a compilation of DET wave functions of near Hartree-Fock quality for all the neutral and singly-charged atoms with the number of electronsN≤54.

The influence of the heat treatments on the nitriding of steels

Extensive experiments have allowed to assess the influence of previous treatments upon the hardness characteristics of UNI 41CrAlMo7 and UNI 45CrMo5 steels nitrided using three atmospheres with increasing nitrogen fugacity. Normalizing, normalizing plus quenching and tempering, normalizing plus double quench and tempering were tested. Samples solely normalized exhibited the largest degree of surface hardening, whereas double quench and temper caused the most limited hardening effect more visible in the case of the A1 free steels. The latter steel is more sensible to nitrogen fugacity increases. The interrelation between the degree of carbide formation in the matrix during prior massive heat treating and the consequent matrix depletion of nitride forming elements has been seen as the cause of the different hardnesses determined after various massive and surface heat–treating combination.

Fatigue of Autofrettaged Thick Tubes: Effect of Thermal Shock, Electropolishing, and Tempering

Thick tubes of 32.3 mm (1.27 in.) ID and wall ratio of 2.5 for SAE 4333 steel were subjected to fatigue testing after the following treatments: normal autofrettage followed by thermal shock; electropolishing; electropolishing then normal autofrettage; electropolishing then normal autofrettage then thermal shock; electropolishing then open-ended autofrettage; tempering from Rc 36 down to Rc 29. Results showed a thermal shock of 600°C for 3 s reduced the fatigue strength of autofrettaged specimens 6 to 8 percent. Electropolishing reduced the fatigue strength 13 percent for as-received bores and 6 percent for autofrettaged bores. Double tempering to Rc 29 reduced the fatigue strength 57 percent compared to Rc 36.

The composition of precipitated austenite in 5.5ni steel

Both scanning transmission electron microscopic (STEM) and chemical extraction techniques were used to analyze the chemical content of precipitated austenite in 5.5Ni steel as a function of heat treatment. Austenite was introduced into the steel by tempering at 600 °C for 2 h (QT2) or 100 h (QT100), by tempering for 1 h at 670 °C (QL), or by double tempering 1 h at 670 °C + 1 h at 600 °C (QLT). The two methods of chemical analysis employed for the analysis of this austenite differ quantitatively in the measured austenite composition but are in qualitative agreement; they show an austenite enrichment in Ni, Cr, Mn, and Mo which is most pronounced (and nearly equivalent) for the QT100 and QLT treatments. The similar enrichment of the QT100 and QLT material is interpreted in light of the sequence of reactions leading to the QLT structure. A good correlation is found between the apparent solute enrichment of the precipitated austenite and its thermal stability on cooling to 77 K.

The improvement of Sulfide Stress Cracking Resistance of 12% Chromium Stainless Steels Through Heat Treatment

Abstract The effect of heat treatment on the sulfide stress cracking (SSC) resistance of CA-6NM, CA-15, and AISI 410 stainless steels were evaluated using uniaxially stressed tensile specimens and notched C-ring specimens. The embrittling environment was an aqueous solution containing 5% NaCl, 0.5% acetic acid saturated with H2S at room temperature. It was observed that the susceptibility of these alloys to SSC was dependent on yield strength and microstructure, which could be controlled by heat treatment. Specifically, the SSC resistance of CA-6NM was significantly improved after a double tempering treatment of 677 C followed by 607 C as compared to a CA-6NM alloy which was only single tempered at 607 C. At equivalent yield strength levels, CA-6NM was less susceptible to SSC than CA-15 or 410, although it was not immune. The effect of heat treatment on the SSC embrittlement phenomenon in these alloys was rationalized on the basis of microstructural changes occurring during tempering.

１０Ｃｒ‐７Ｃｏ‐Ｍｏ‐Ｗ‐Ｖ系耐熱鋼製ロータディスクの基礎特性 Ｉ

The fundamental properties, chemical composition and heat treatment of 10Cr-7Co-Mo-W-V heat resisting steel used as rotar material of gas turbine were studied. And the creep rupture and impact properties of products based on the fundametal study were also studied. The results obtained are as follows.(1) The creep rupture strength and impact vaiue of 10Cr-7Co-Mo-W-V steels, depending upon the factor α=Nb+/8(C+N)2, shows the maximum value when α=0.95∼1.10.(2) 10Cr-7Co-Mo-W-V steels are usually produced through quenching and double tempering. When subzero treated between the first and second tempering, the impact value is remarkably improved.(3) In a practical test the creep rupture strength of this steel is the same as that of H53 and it is superior to H46 and AISI 422. Further, the impact value is superior to that of H46 and AISI 422.(4) The anisotropy of mechanical properties is little.(5) When the material above mentioned is heat treated under the optimum condition, the microstructure is tempered martensite (containing no δ-ferrite).

Effect of double tempering on the properties of steels

1. Preliminary heating may have a substantial effect on the structure and properties of quenched steels subsequently tempered at higher temperatures. 2. A high impact strength in the range of irreversible temper brittleness without reduction of the strength characteristics can be obtained by selecting the optimal parameters of double tempering for steels alloyed with elements inhibiting the decomposition of martensite.