Impact Transition Temperature Research Articles

Effects of Intercritical Heat Treatment on the Temper Embrittlement of SA508 Gr.4N Ni-Cr-Mo High Strength Low Alloy Steels for Reactor Pressure Vessels

To analyze the effects of intercritical heat treatment on the temper embrittlement of SA508 Gr.4N steels, two model alloys with different phosphorus (P) contents were fabricated. Each sample was heat treated by applying a conventional heat treatment process (quenching-tempering) with/without an intercritical heat treatment process (IHT) and a step-cooling heat treatment for temper embrittlement. Then their microstructure and mechanical properties were evaluated. The microstructure of the SA508 Gr.4N model alloy was composed of tempered lower bainite and martensite, and nano-sized precipitates formed both inside and at boundaries. The grain size was reduced when IHT was applied. There was a small difference in tensile properties according to the heat-treatment conditions and P contents, but the difference in Charpy impact properties was large. The heat treatment for temper embrittlement (TE) increased the impact transition temperature, and a very significant increase was observed in steels with a high P content. The increase in transition temperature owing to TE was reduced when IHT was applied. The fractograph analysis of Charpy fractured specimens at transition temperatures showed that an increase in intergranular fracture was main reason for the TE, and that IHT reduced the formation of intergranular fracture. The AES results showed that P-Ni was segregated at grain boundaries, and the level of segregation was reduced by applying IHT. This occurred because the formation of prior austenite grain boundaries by IHT dispersed the P at grain boundaries, and reduced the amount of P segregation.

METALLURGICAL AND MECHANICAL INVESTIGATION OF TIG ARC WELDMENTS FOR API 32″ GAS PIPE

API steel gas pipe of 32" diameter and 15.9 mm thickness was investigated to characterize the steel grade and assess weldability for fourteen-consecutive passes of TIG arc welding technology. The parent Metal (PM) contains 0.02% C in addition to 0.02% Nb. A steel electrode for TIG process was used containing 5.6% Cr and 0.56% Mo. Standard V-groove was mechanically prepared to suite butt welding. The microstructure of the PM was containing mainly fine acicular ferrite satisfying the essential requirements of API specifications for grade X60. The weld metal (WM) was containing very fine acicular ferrite and showing the maximum hardiness values. Heat affected zone (HAZ) structure still contains acicular ferrite but it became coarse, possessing lower hardness than that of WM. HAZ does not exceed 2-2.5 mm adjacent to the welded V-groove. The fractured tensile welded specimens were necked and failed at the HAZ. The impact transition temperature (ITT) was detected as -35 oC. However, the mechanical properties of the WM did not negatively affected by the welding process and still fair satisfying the requirements of API X60.

Toughness Property Control by Nb and Mo Additions in High-Strength Quenched and Tempered Boron Steels

The synergetic effect on hardenability by combining boron with other microalloying elements (such as Nb, Mo and Nb + Mo) is widely known for high-strength medium carbon steels produced by direct quenching and subsequent tempering treatment. The improvement of mechanical properties could be reached through optimization of different mechanisms, such as solid solution hardening, unit size refinement, strain hardening, fine precipitation hardening and the effect of carbon in solid solution. The current study proposes a procedure for evaluating the contribution of different microstructural aspects on Charpy impact toughness. First, the effect that austenite conditioning has on low-temperature transformation unit sizes and microstructural homogeneity was analysed for the different microalloying element combinations. A detailed crystallographic characterization of the tempered martensite was carried out using electron backscattered diffraction (EBSD) in order to quantify the effect of unit size refinement and dislocation density. The impact of heterogeneity and presence of carbides was also evaluated. The existing equations for impact transition temperature (ITT50%) predictions were extended from ferrite-pearlite and bainitic microstructures to tempered martensite microstructures. The results show that microstructural refinement is most beneficial to strength and toughness while unit size heterogeneity has a particularly negative effect on ductile-to-brittle transition behaviour. By properly balancing alloy concept and processing, steel having a yield strength above 900 MPa and low impact transition temperature could be obtained by direct quenching and tempering.

Uncertainty quantification and composition optimization for alloy additive manufacturing through a CALPHAD-based ICME framework

During powder production, the pre-alloyed powder composition often deviates from the target composition leading to undesirable properties of additive manufacturing (AM) components. Therefore, we developed a method to perform high-throughput calculation and uncertainty quantification by using a CALPHAD-based ICME framework (CALPHAD: calculations of phase diagrams, ICME: integrated computational materials engineering) to optimize the composition, and took the high-strength low-alloy steel (HSLA) as a case study. We analyzed the process–structure–property relationships for 450,000 compositions around the nominal composition of HSLA-115. Properties that are critical for the performance, such as yield strength, impact transition temperature, and weldability, were evaluated to optimize the composition. With the same uncertainty as to the initial composition, and optimized average composition has been determined, which increased the probability of achieving successful AM builds by 44.7%. The present strategy is general and can be applied to other alloy composition optimization to expand the choices of alloy for additive manufacturing. Such a method also calls for high-quality CALPHAD databases and predictive ICME models.

Evaluation of Transition Temperature in Reactor Pressure Vessel Steels 6using the Fracture Energy Transition Curve from a Small Punch Test

The small punch (SP) test is one of the small specimen test techniques, and standardization of the SP test method for evaluating the mechanical properties of metallic materials is in progress. In this study, the impact transition temperature of reactor pressure vessel steels (RPV) in nuclear power plants was estimated using the draft standard SP test method. The SP fracture energy (ESP) and normalized SP fracture energy (ENSP) of the RPV steels were evaluated at various temperatures, and their transition curves were derived and compared to the transition curve in the Charpy V notch (CVN) test. The SP transition region appeared at a much lower temperature range than that of the CVN owing to the size and notch effect. Ductile brittle transition temperature (DBTT) in the SP transition curve showed a linear relationship with DBTT and T41J in the CVN transition curve. The ductile to brittle transition behaviors of SP specimens were analyzed using fractographs and compared with the transition curves in ESP and ENSP. ENSP started to decrease at the temperature at which the SP ductile to brittle transition behavior occurred, and this means that the ENSP transition curves were in good agreement with transition behavior in the SP test. However, the ESP transition curves did not match transition behavior. Using DBTT in the ENSP transition curve is appropriate to estimate the CVNDBTT and T41J.

Fitting curves and impact toughness transition temperature of quenched and tempered steel welds

Welded products of quenched and tempered (QT) structural steels are used from ambient to moderately low design/service temperatures. Therefore, besides base metal, a weld also must possess required minimal design impact toughness (KV) at temperatures lower than transition temperature (TT), where the transition from ductile to brittle fracture occurs. A common way to determine transition temperature is by use of appropriate fitting curves, in accordance with specified standardised criterion. From the point of welding procedure evaluation, it is important to analyse welds for its impact toughness and transition temperature, particularly for three main zones: weld metal, heat-affected zone (HAZ) and base metal. This paper covers welds of two QT steel grades, 690 and 890, which are interesting regarding their yield strength and characteristic weld zones. Basic details of gas metal arc welding (GMAW, process used in this experiment) are provided, while temperatures for impact toughness tests of weld zones were varied from +20 °C down to -60 °C. Based on acquired experimental results of impact toughness, fitting curves were developed by use of Oldfield model, e.g. hyperbolic tangent function. Acquired transition temperatures (TT) from fitting curves show mostly allowable values for all three weld zones. As expected, lower strength grade 690 possess higher impact toughness, in comparison to higher strength grade 890. The standardized criteria of minimal absorbed energy of 30 J (KV) and 50 % of shear fracture (SF) show different transition temperatures (TT-30J and TT-50%SF), while general dependence of impact toughness to shear fracture (KV vs. SF) shows a reasonable trend. Finally, used GMAW procedures may be considered as acceptable, since for both steel grades (690 and 890) all three weld zones show better TT-30J values than minimal required by standard (TT-30J=-40 °C) for QT structural steels.

The influence of a high aluminium addition on the strength and impact behaviour of hot-rolled Nb containing steels

An addition of 0.3 wt-% Al has been made to a Nb containing 0.1 wt-% C, 1.4 wt-% Mn steel in an attempt to improve its impact behaviour on hot rolling to 15 mm thick plate; the intention being to replace some of the specifications that currently can only be achieved by the more expensive methods of control rolling or normalising. Previous work had shown that a 0.2 wt-% Al addition to a hot-rolled plain C–Mn steel resulted in the impact transition temperature decreasing by 40°C with no significant change in strength making such a proposition possible. Unfortunately, the coarse austenite grain size of the hot-rolled Nb containing steel resulted on transformation in colonies of lower transformation products and Widmanstätten ferrite forming in the ferrite/pearlite microstructure resulting in poor impact behaviour. Reducing the cooling rate by working with thicker plate, 30 mm, led to little change in the mechanical behaviour on hot rolling. In contrast, on control rolling, the finer-grained steels which were virtually free of these lower transformation products showed a small benefit from the high Al addition even when Nb was present, particularly for the slower cooling rate, the 27J impact transition temperature decreasing by ∼20°C together with a small increase in yield strength. Further work is suggested to decrease the hardenability so that the original aim might be achieved.

On the Fracture Behavior of a Creep Resistant 10% Cr Steel with High Boron and Low Nitrogen Contents at Low Temperatures.

An advanced, high chromium, creep-resistant steel was subjected to the tensile tests and three-point bending tests of Charpy V-notch specimens at temperatures of −196 to 20 °C. The steel exhibited ductile fracture under tension tests at all of the temperatures studied. The mechanical properties, i.e., strength and uniform elongation, were enhanced with a decrease in temperature down to −140 °C. Transgranular, dimpled fracture remained the primary fracture mechanism under tension. On the other hand, the results obtained with Charpy V-notch specimens suggested the ductile–brittle transition (DBT). Full embrittlement was observed at temperatures of −60 °C and −150 °C upon impact tests and three-point bending tests, respectively, when the unstable crack started to propagate without remarkable plastic deformation. The DBT temperature of −27 °C for the present steel corresponded to the 28 J impact transition temperature, T28J, when the maximum impact stress matched the maximal true tensile stress.

Splitting susceptibility in modern X80 pipeline steels

The microstructure and crystallographic texture of modern industrially produced API X80 grade pipeline steels were studied to identify the factors associated with the phenomenon known as splitting, which can occur on the fracture surfaces of Charpy, drop-weight tear test and full-scale fracture propagation tests. The steels investigated originated from full-scale production of line pipes, which had shown different levels of crack arrestability under full-scale burst test conditions. It was shown that a low Charpy impact transition temperature in the through thickness direction corresponded with a low susceptibility to splitting under conventional test methods and an increased resistance to ductile fracture propagation. The microstructural features shown to be consistent with an increased susceptibility to splitting included banding of the {001}<110> texture component in the microstructure and the presence of hard microstructure constituents being potential initiation sites for cleavage fracture. It is suggested that these results provide a viable test method to classify steel susceptibility to splitting and potential performance in full-scale ductile fracture tests.

Charpy impact toughness and transition temperature in ferrite–perlite steel

Las pruebas de impacto son una importante herramienta para la obtención de importantes propiedades, como la tenacidad al impacto y la temperatura de transición ductil-fragil, para la selección de un material en diseño mecánico. Se evaluó la tenacidad al impacto mediante pruebas de impacto Charpy en acero de fase doble (ferrita-perlita) de 10 mm de espesor a temperaturas que oscilan entre 90 ºC y -60 ºC. Las superficies de fractura fueron analizadas por microscopio óptico (MO). Debido a la alta dispersión, mostrada en los resultados, se emplaron numerosas pruebas en las mismas condiciones y el uso de métodos estadísticos para obtener un valor confiable. La temperatura de transición calculada (45 ºC) y la resistencia al impacto de Charpy se analizaron en función de la microestructura y el equivalente de carbono

Investigation of the Microstructural, Mechanical and Corrosion Properties of Grade A Ship Steel-Duplex Stainless Steel Composites Produced via Explosive Welding

Grade A ship-building steel-AISI 2304 duplex stainless steel composite plates were manufactured via explosive welding. The AISI 2304 plates were used to clad the Grade A plates. Optical microscopy studies were conducted on the joining interface for characterization of the manufactured composite plates. Notch impact, tensile-shear, microhardness, bending and twisting tests were carried out to determine the mechanical properties of the composites. In addition, the surfaces of fractured samples were examined by scanning electron microscopy (SEM), and neutral salt spray (NSS) and potentiodynamic polarization tests were performed to examine corrosion behavior. Near the explosion zone, the interface was completely flat, but became wavy as the distance from the explosion zone increased. The notch impact tests indicated that the impact strength of the composites decreased with increasing distance from the explosion zone. The SEM studies detected brittle behavior below the impact transition temperature and ductile behavior above this temperature. Microhardness tests revealed that the hardness values increased with increasing distance from the explosion zone and mechanical tests showed that no visible cracking or separation had occurred on the joining interface. The NSS and potentiodynamic polarization tests determined that the AISI 2304 exhibited higher corrosion resistance than the Grade A steel.

Impact Properties of Austempered Spheroidal Graphite Cast Irons

This study aims to clarify the influence of external notch on impact characteristics of high toughness ductile cast irons prepared by austempering heat treatment. We produced ductile cast irons samples with various matrix microstructure tested by Charpy impact within five kinds of external notches whose stress concentration factors (α), with values taken from 1.0 (Un-notched) to 4.8. In addition, to clarify the initiation process of impact characteristics, we observed the evolution of microstructure surface during bending tests with a slow loading speed for the un-notched and the notched impact samples. The results showed that the impact fracture energy decreases strongly in the range of α from 1 to 2.3 but decreases slightly for α larger than 3. Moreover, the impact value of samples with austempered microstructure is sensitive to the external notch shape. The impact transition temperature increases with increasing the stress concentration factor. The fracture energy is decreasing with the external notch from the impact test since the crack initiation energy is directly affected by this later. This work contributes to get a better understanding in the basic theories of external notch effect on impact characteristics of austempered spheroidal graphite cast irons (ADI).

Effect of Thermal Exposure on the Charpy Impact Properties of Thermo-Mechanically Treated Reinforcement Steel Bar

The reinforcing bars provide necessary structural support for the buildings made with concrete under normal and demanding conditions. The two most structurally demanding situations in building can arise under seismic activities and fire. This paper deals with the behavior of normal Fe 500D (ASTM 1786) rebars under fire situations. The thermo-mechanically treated (TMT) 12 mm rebar samples are heat-treated at different temperatures (150–650 °C for 3 h) and are subjected to microstructural, textural characterization, and tensile and Charpy impact testing. Experimental results show that reheating the rebars below 550 °C does not cause much deterioration in its properties. However, when the same is heated above 550 °C, the temper martensite structure at the rim and mixed ferrite-pearlite-bainite structure at the core region of the rebar is converted into coarse polygonal ferrite grains and carbide particles. These not only reduce the strength, but also significantly increase the impact transition temperature of the TMT rebar. The intensification of detrimental cube and Goss texture with the expense of beneficial gamma-fiber texture and the release of beneficial compressive residual stress upon reheating treatment can also hamper the impact transition behavior of the heat-treated rebar.

Dynamic fracture behaviour of thermo-mechanically processed modified 9Cr–1Mo steel

Samples from normalized and tempered modified 9Cr–1Mo steel, were normalized at three different temperatures and rolled at four different temperatures and subsequently tempered. Samples were then subjected to instrumented Charpy impact tests. Impact properties were represented in terms of upper shelf energy, ductile-to-brittle transition temperature, 28J energy transition temperature, general yield temperature, blunt-notched dynamic fracture toughness (Kjd) and reference temperature. Kjd was determined following the modified Schindler’s procedure. Correlation between different transition temperatures and their conservativeness were analyzed in view of fracture safe design. Finally, microstructural parameters were related to the impact transition temperatures based on microstructural and fractographic studies.

Heat affected zone microstructures and their influence on toughness in two microalloyed HSLA steels

Microstructures and Charpy impact properties have been examined in two microalloyed steels following heat treatments to simulate weld heat affected zone (HAZ) structures over a range of heat input conditions, characterised by the cooling time from 800 to 500°C (Δt8/5). The base materials were low carbon structural steel plates microalloyed with vanadium and nitrogen (V–N) and niobium (Nb), respectively. The toughnesses of the HAZs displayed remarkably different behaviours as shown by their impact transition temperatures. For the V–N steel, the toughness improved with increasingly rapid cooling (low heat input conditions) whereas the Nb steel showed an opposite trend. Some of this behaviour could be explained by the presence of coarse ferrite grains in the slowly cooled V–N steel. However, other conditions where all the structures were bainitic and rather similar in optical micrographs gave widely different toughness values. The recently developed method of five dimensional boundary analysis based on electron backscattering diffraction has been applied to these cases for the first time. This showed that the lath boundaries in the bainite were predominantly on {110} planes of the ferrite and that the average spacing of these boundaries varied depending on steel composition and cooling rate. Since {110} is also the slip plane in ferrite, it is considered that close spacing between the lath boundaries inhibits general plasticity at stress concentrations and favours initiation of fracture. The differences between the two steels are believed to be due to their transformation behaviours on cooling where precipitation of vanadium nitride in austenite accelerates ferrite formation and raises the temperature of the phase transformation in V–N steels.

Effect of microstructural parameters, microtexture and matrix strain on the Charpy impact properties of low carbon HSLA steel containing MnS inclusions

Low-carbon micro-alloyed steel containing coarse MnS inclusions was subjected to different thermo-mechanical processing routes to evaluate the effect of microstructure on its Charpy impact properties over a range of temperatures. MnS inclusions were found to deteriorate the upper shelf energy (USE) of the steel but its effect on the impact transition temperature was not as detrimental as that due to the presence of TiN particles. MnS inclusions were responsible for the initiation of micro-voids; however, the propagation of the cracks from these depends on the effective grain size and the strength of the matrix. An increase in the density of dislocations and low-angle boundaries enhances the strength and the strain-hardening ability of the ferrite matrix. This helps in retarding the growth of micro-voids, thereby reducing USE and promoting cleavage crack propagation resulting in an increase in its impact transition temperature. Refinement of effective grain size, on the other hand, increases the crack propagation resistance and therefore, improves the low-temperature toughness of the steel. Finish rolling of the steel just above the Ar3 temperature (austenite to ferrite transformation start temperature) or a simple normalizing treatment of the as-rolled plates at a low austenitization temperature is recommended from the point of view of higher impact toughness and lower impact transition temperature.

Effect of TiN Particles and Grain Size on the Charpy Impact Transition Temperature in Steels

The toughness of ferritic steels is influenced by the grain size distribution, second phase, precipitates and coarse inclusions. In this work an examination of the effect of coarse TiN particles (>0.5 μm) and ferrite grain size on the Charpy impact transition temperature in high strength low alloyed steels has been carried out. Steels with high Ti content (up to 0.045 wt%), have been heat-treated and furnace cooled to obtain a ferrite-pearlite microstructure with different ferrite grain sizes. Coarse TiN particle size and ferrite grain size distributions have been measured and Charpy impact testing has been carried out. Scanning electron microscopy (SEM) analysis has been used to measure the grain boundary carbide thickness and to determine if the coarse TiN particles are acting as cleavage initiation sites by fractographic analysis. The Charpy ductile-brittle transition temperatures (DBTT) have been predicted using standard literature equations, and compared to the measured values. The relationship between the ferrite grain size and coarse TiN particle size and number density in terms of whether the coarse TiN particles act as effective cleavage initiation sites is discussed in this paper.

Origin and Propagation of Splits in High-Strength Low-Alloy Strip Steel

A high-strength strip steel (yield strength of ~700 MPa) with a ductile-brittle transition over a wide temperature range shows splits (fissures) on the fracture surfaces in the upper transition region. The steel, hot rolled to strip thicknesses of approximately 10.0 mm and 16.8 mm, had a predominantly fine-grained ferrite microstructure with some coarse grain patches (area percent of 11 pct and 42 pct, respectively). Low-blow Charpy tests were carried out at room temperature, corresponding to the upper transition region for these strips. The low-blow tests resulted in the formation of splits without main crack propagation from the notch; therefore, the energies at which the splits initiated could be determined. Acoustic emission (AE) sensors were used during low-blow Charpy testing of the strip steels and mild steel (where no splits occur); it was found that AE was able to detect signals from the hammer impact, split formation, and ductile deformation. Optical microscopy, scanning electron microscopy (SEM), and X-ray tomography were carried out, which verified the presence of splits and showed that they propagated by transgranular cleavage, preferentially following coarse-grained regions. No significant difference in the number or length of splits between the 10- and 16.8-mm strip was observed, but the 10-mm strip did produce deeper splits during the low-blow Charpy testing. The deeper splits contribute to a lower impact transition temperature for the 10-mm strip material.

Influence of directionality on strength and impact behaviour of high strength steels

The directionality of the strength and impact behaviour of control rolled 110 grade high strength steel has been examined. Tensile specimens tested along the rolling direction were found to give strengths of ∼60 MPa lower than when tested transversely in the rolling direction. Charpy V notch impact samples, in contrast, gave lower impact transition temperatures when taken in the rolling direction (35°C lower). The difference in strength between the two directions is believed to be mainly as a result of the strong texture produced on control rolling. The difference in impact behaviour is due to the more severe fissuring on the fracture surface of the longitudinally tested specimens. Fissuring is more marked on the longitudinally tested sample because of the greater grain elongation in that direction, the initial fracture path being along the grain boundaries.

A Critical Grain Size Concept to Predict the Impact Transition Temperature of Ti-Microalloyed Steels

Based on the assumption that local principal stress remains the same everywhere within a ferrite grain, a critical value of grain size can be determined for a fixed TiN particle size. When the grain size is smaller than the critical size, grain boundary is expected to resist the propagation of a micro-crack that is initiated from a TiN particle. Using this concept, an attempt has been made to predict the local cleavage fracture stress and 27J impact transition temperature (ITT) of different Ti-microalloyed steels, which were subjected to (instrumented) Charpy impact testing.