Impact Toughness of Low Alloy Steels Subjected to Heat Treatment and Equal-Channel Angular Pressing at Low Temperature

The paper deals with the impact toughness and fracture mode of the low-alloy 09G2S steel in the as-delivered state and in the condition after equal-channel angular pressing (ECAP) and annealing at impact bending in the temperature range of 293-203 [K]. The results on impact toughness KCV and fracture mode of Charpy test specimens made of the 09G2S steel in the as-delivered state and in the condition after hardening are presented. There is a decrease in the steel impact toughness in the examined temperature range after annealing at 673 [K], ECAP, and ECAP with annealing compared to the corresponding value for steel in the as-delivered state. For the 09G2S steel treated with various techniques, the temperature-KCV plot drop occurs smoothly, without abrupt changes, in the examined temperature range, in contrast to the behavior of this value with the upper threshold of the fracture appearance transition temperature for the steel in the as-delivered state.

Effect of PWHT Conditions on Toughness and Creep Rupture Strength in Modified 9Cr-1Mo Steel Welds

We clarified the effect of post weld heat treatment (PWHT) conditions on the toughness and creep rupture strength of modified 9Cr–1Mo steel weldments used for high temperature components of ultra-supercritical power plants. Fracture appearance transition temperature (FATT) decreased as PWHT temperature increased, and for all of the weld metals of tungsten inert gas welding, submerged arc welding and shielded metal arc welding, FATTs were lower than 293 K when the PWHT temperature was higher than 1,008 K. In contrast, in the uniaxial creep test with a loaded stress of 108 MPa, the creep rupture strength of the specimen on which PWHT was carried out for a holding time of 7.2 ks was significantly decreased when the PWHT temperature was more than 1,033 K. Therefore, the appropriate PWHT temperature range to maintain the toughness and creep fracture strength was 1,008 K ≤ T ≤ 1,033 K.

EFFECT OF QUENCHING AND TEMPERING PROCESS ON A MEDIUM C STEEL WITH LOW CHROMIUM AND MOLYBENUM ADDITION FOR FORGED COMPONENTS

<p class="AMSmaintext"><span lang="EN-GB">In this paper the effect of quenching and tempering (Q&amp;T) thermal treatment on mechanical properties of a C-Mn steel with 0.22% Cr for forged components is studied. Due to the lack od any micro-alloying elements (such as vanadium or niobium) such steel can just reach mechanical target allowed by its intrinsic hardenability. Aim of this work is to evaluate the mechanical properties dependence as a function of different quenching and tempering treatments. Results show that, after Q&amp;T, steel can reach a yield strength of 330 MPa combined with a -20°C </span><span lang="EN-GB">fracture appearance transition temperature (50% FATT) measured with a Charpy-V impact test making this steel suitable for low temperature application.</span></p>

Metallographic and Materials Evaluation of a Cracked Radial Steam Turbine Rotor

A radial steam turbine developed cracks after 220,000 hours of service. The rotor had an integral disc with eight rows of blades, and a short stub. Nine inlets on the disc channeled steam from one side to the other, and then radially outward. Analysis of the fracture surface revealed cracks originating in some of the inlet holes, and propagating by fatigue. No material defects were found at the crack initiation sites. Hardness and microstructure (optical) across the disc were uniform, but chemical composition analysis of the alloy revealed high level of phosphorus and sulfur. In addition, the microstructure consisted of uniformly tempered martensite with manganese sulfide stringers. Although tensile properties were normal, impact testing indicated embrittlement by a shift in Fracture Appearance Transition Temperature (FATT). Metallurgical evidence of embrittlement was also found. It was concluded that service induced cyclic loading in combination with reduced crack resistance caused by embrittlement lead to cracking.

Effect of Reheating and Deformation Processes on DWTT Performance for Heavy Gauge Pipeline Steel

DWTT (Drop weigh tear test) is an effective way to evaluate the fracture propagation for pipeline steel. The effects of slab reheating temperature, soaking time, single pass reduction ratio during recrystallization zone rolling and transfer bar ratio during non-recrystallization zone rolling on DWTT performance were studied for heavy gauge pipeline steel. And the grain refinement and toughening mechanism were discussed. It was found that the grain in the core of the plate can be refined by reducing the reheating temperature, increasing the single pass reduction ratio during recrystallization zone rolling and setting suitable transfer bar ratio during non-recrystallization zone rolling, which promote the DWTT property improvement for heavy gauge pipeline steel. The 30.9mm heavy gauge pipeline steel plate was industrial produced and the X70 UOE welded pipe with dimension in Φ1219×30.9mm was manufactured. The DWTT 85%FATT (fracture appearance transition temperature) of pipe body is as low as -20°C.

Utilization of SPT Techniques for Evaluation of Changes in the Properties of the VVER-440 Reactor Pressure Vessel Steels After Their Irradiation in the Research and Power Reactors

The paper describes the testing procedures and the basic results of the evaluation of the Small Punch Test (SPT) specimens after their irradiation in the Halden reactor in Norway. The SPT technique was used for estimation of basic mechanical properties as ultimate tensile strength and yield stress of the tested materials as well as the Fracture Appearance Transition Temperature (FATT). The main aim of the work was to compare the SPT results obtained from the surveillance specimen programs implemented in the Slovak power reactors with the SPT results from the specimens irradiated in the research reactor in Halden. For the project there were chosen 3 types of steels used for construction of the reactor VVER 440/213 type in Bohunice NPPs in the Slovak Republic. The experimental materials were two bainitic steels — base metal and weld metal of the reactor pressure vessel wall and austenitic cladding of the reactor wall. Two sets of SPT specimens together with mini-tensile specimens prepared from the experimental materials were irradiated in the Halden reactor. The samples were irradiated at 275°C to two fluence values which are equivalent to approximately of 4 and 6 campaigns in the power reactor. Obtained results are compared to up-to-date SPT results from the surveillance specimen program applied at Bohunice NPP in the Slovak Republic.

Effect of Microstructure on Mechanical Behavior of a Combined Steam Turbine Rotor Steel

High-pressure low-pressure (HLP) combined rotors have been gradually used in advanced power plants. In the present work, tensile, impact, fatigue and creep experiments were conducted to comprehensively investigate mechanical properties of a newly developed combined rotor steel 25Cr2NiMo1V based on microstructure influence. Fatigue crack growth (FCG) rates were obtained by both constant amplitude method and the load-shedding technique. A new method based on cyclic plastic zone size being equalling to grain size was introduced to defferentiate corresponding FCG data in the Paris regime and the near-threshold regime. Results show that 25Cr2NiMo1V steel has sufficient lower temperature strength and toughness in LP zone, and good high temperature creep properties in HP zone. Fracture Appearance Transition Temperature (FATT) at center core of LP is lower than 3°C with a tensile strength of 850 MPa, and creep rupture strength of HP for 105 h is respected to reach 165 MPa at 566°C. By comparison with other combined rotor materials in literature, the properties of 25Cr2NiMo1V steel enable it particularly suitable to HLP rotor material for advanced combined cycle power plants.

Evaluation of Temper Embrittlement of 30Cr2MoV Rotor Steels Using Electrochemical Impedance Spectroscopy Technique

Temper embrittlement tends to occur in the turbine rotor after long running, which refers to the decrease in notch toughness of alloy steels in a certain temperature range (e.g., 400°C to 600°C). The severity of temper embrittlement must be monitored timely to avoid further damagement, and the fracture appearance transition temperature (FATT50) is commonly used as an indicator parameter to characterize the temper embrittlement. Compared with conventional destructive methods (e.g., small punch test), nondestructive approaches have drawn significant attention in predicting the material degradation in turbine rotor steels without impairing the integrity of the components. In this paper, laboratory experiments were carried out based on a nondestructive method, electrochemical impedance spectroscopy (EIS), with groups of lab-charged specimens for predicting the temper embrittlement (FATT50) of turbine rotor steel. The results show that there was a linear relationship of interfacial impedance of the specimens and their FATT50values. The predictive error based on the experiment study is within the range of ±15°C, indicating the predicting model is precise, effective, and reasonable.

Microstructure-Mechanical Properties Relationships for Complex Microstructures in High Strength Steels

Due to the increased complexity of steel microstructures, when considering the application of available Hall-Petch type equations for yield strength prediction, a number of difficulties raises. For example, the correlation between grain size measurements by EBSD technique and optical microscopy (OP) in complex microstructures is required in order to integrate data to the traditional equations developed for OP results and ferrite-pearlite microstructures. Besides, the introduction of some additional terms to the equations to account for precipitation, C in solution and forest dislocation contributions presents some difficulties that need to be overcome to improve prediction accuracy. Different microstructures (ferrite-pearlite, bainite, quenched and Q&T) have been produced by thermal and thermomechanical treatments, followed by microstructural characterisation and mechanical testing. A Hall-Petch coefficient dependent on the boundary misorientation distribution is proposed. This approach allows dealing in a similar way ferritic, bainitic and martensitic microstructures. The Hall-Petch coefficient, thus defined, corresponds to the previously proposed by Pickering for ferrite, while bainitic microstructures give a smaller value. Additionally, the equation used to express the fracture appearance transition temperature of ferritic-pearlitic microstructure has been generalized from the developments made in the calculation of the yield stress.

Effects of Cooling Rate on Impact Toughness of an Ultrahigh-Strength TRIP-Aided Martensitic Steel

The effects of variations in the rate of post-austenitization cooling of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.2%Mo-1.5%Ni-0.05%Nb (mass%) transformation-induced plasticity (TRIP)-aided steel with a lath martensite structure matrix on the Charpy impact toughness were investigated, with the aim of improving the material properties for automotive body applications. When cooled at 1.2°C/s after austenitization, the TRIP-aided steel showed a higher upper-shelf Charpy impact absorbed value (90 J/cm2) and a lower ductile-brittle fracture appearance transition temperature (−126°C), compared with the values determined (82 J/cm2, −98°C) for steel cooled at 53.5°C/s. The lower cooling rate yielded a higher volume fraction and carbon concentration of metastable retained austenite, finer martensite-austenite constituents, and a lower carbide fraction in the wide lath martensite structure in the TRIP-aided steel. These improved microstructural characteristics resulted in superior impact toughness.