An intensive quenching (IQ) process is an environmentally friendly method of hardening steel parts. Digitally controlled, IQ employs highly agitated and directed water flow as the quenchant. An extremely high cooling rate applied uniformly over the entire part surface area induces high surface compressive stresses which prevents part distortion and cracking while forming a very fine microstructure. The fine microstructure results in better mechanical properties compared to properties imparted by conventional oil or polymer quenching. The improved mechanical properties enable engineers to design stronger steel parts for higher power density mechanical systems often using steels containing a less amount of alloying elements or using less expensive plain carbon steels. A broad and deep body of knowledge documents IQ’s ability to tailor a steel component’s microstructure to improve steel parts mechanical properties and performance. A sampling of data will be presented including surface and core hardness, tensile, yield and impact strength, elongation and reduction in area, residual surface compressive stresses for through hardened steels and the carburized grades. IQ systems can be readily “dropped in” to existing steel processing facilities or integrated into next generation heating and cooling systems through teamed relationships with equipment makers and part manufacturers seeking a sustainable future.

One of the methods of evaluating the mechanical properties of a material in the case of its limited amount is the use of techniques that employ the miniaturized test specimens. The basic properties used mostly for residual life evaluation are tensile strength, impact notch toughness or impact notch toughness transition curve, fracture toughness, creep and high cycle fatigue. For example, by semi-destructive sampling of operating power equipment, actual material properties can be obtained which are crucial for predicting the residual life of the equipment. Furthermore, the local material properties of the weld joint in individual zones can be determined. In this paper applicability of these test methods is described, specific examples of use are given and reference is made to the existing ISO/ASTM 52909:2022 standard for the use of sub-size samples.

Hydrogen embrittlement (HE) susceptibility was investigated for Alloy 718 and Alloy 945X specimens heat treated to a set of conditions within the specifications of API Standard 6ACRA. Heat treatments were selected to simulate the potential variation in thermal history in thick sections of bar or forged products and produce various amounts of discontinuous grain boundary δ phase in Alloy 718 and M 23 C 6 carbides in Alloy 945X, while maintaining a constant hardness in the range of 35-45 HRC for Alloy 718 and 34-42 HRC for Alloy 945X. Time-temperature-transformation (TTT) diagrams and experimentation were used to select a set of heat treatments containing no δ phase, a small quantity of δ, and a larger quantity of δ in Alloy 718. The presence of δ phase has not been verified for the moderate condition. A similar approach was taken regarding M 23 C 6 carbides in Alloy 945X. Incremental step loading (ISL) tests were conducted under in-situ cathodic charging on circular notch tensile (CNT) specimens in a 0.5 M H2SO4 solution. During the test, the direct current potential drop (DCPD) was measured across the notch to determine the stress intensity associated with unstable crack growth. Results indicate that even very small quantities of δ phase in Alloy 718 are detrimental to HE resistance. Both Alloy 718 and Alloy 945X show decreases in HE resistance with aging, with a greater degradation in Alloy 718.

Around 1970 it was discovered that quenching AISI 4340 steel from 1200 °C leads to much higher fracture toughness, in the as quenched state, than by conventional austenitizing at 870 °C. Further researches have ascertained that the apparent toughness increase is limited to fracture toughness tests (KIC), whereas Charpy-V impact tests do not show any betterment due to high temperature austenitizing, in respect to conventional heat-treating. Various explanations of these contradicting results were given on the basis of the then existing theories. It was further ascertained that the betterment of fracture toughness was limited upon tempering to a maximum temperature of 250 °C, making it useless for most applications. The puzzling phenomenon has been recently reconsidered for the validation of new Blunt Notch Brittle Finite Fracture Mechanics theories. Results are given and possible future applications to industrial cases presented.

Recent destructive analysis of six ASTM A350 LF2 flanges has revealed vastly different low temperature (-50°F) Charpy impact toughness from 4 J (3 ft-lbs) to greater than 298 J (220 ft-lbs). These relatively low strength flanges, minimum 248 MPa (36 ksi) yield and 483-655 MPa (70-95 ksi) tensile strength, had nominally the same yield and UTS despite the difference in toughness. Detailed chemical and microstructural analysis was undertaken to elucidate the cause of the toughness range. The majority of the flanges had aluminum additions and a fine grain size with the toughness differences mostly explained by the cooling rate after normalizing with the still air cool showing the lowest toughness and the fastest air cooled sample the highest. For flanges of this strength level a quench and temper operation is not required to obtain good low temperature toughness but forced air cooling after normalizing is a minimum cooling rate to ensure good toughness and overall strength.

The effect of forging temperature and temperature before quenching on microstructure is studied. This is related to the mechanical properties like tensile strength, yield strength and impact toughness. It was observed that martensitic needles in direct quenched parts were slightly longer than the normal hardened and tempered parts. This was attributed to the coarser prior austenite grain size, resulting in fewer nucleation sites in case of direct quenched parts.

Usually bainitic microstructures exhibit good toughness and austempering is typically the preferred heat treatment when toughness is the primary requirement of the component. Several reports have shown such characteristics when compared to tempered martensite. High carbon steel may exhibit brittle characteristics but it is a good steel with respect to mechanical properties and wear resistance. The objective of this study was to compare the impact properties of AISI O1, a high carbon tool steel as VND in Brazil. This was done by comparing Charpy impact strength under different heat treatment cycles. Tempered martensite and bainite was obtained at 350°C after holding at temperature for 20, 40, and 60 minutes. Since hardness influences impact behavior, comparative studies were performed at the same surface hardness level. Results show a low absorbed energy for the austempered samples which for this temperature is independent of the holding time.

Austempering, which produces a bainitic microstructure, is widely recognized as the preferred heat treatment when toughness is the primary requirement for a component. The literature contains numerous studies comparing the mechanical properties of tempered martensite with bainite of equivalent hardness. The majority of these findings demonstrate superior properties in bainitic microstructures. Bainite exists in two forms: upper bainite, which forms during isothermal transformation at higher temperatures (typically around 500 °C), and lower bainite, which develops at lower temperatures (approximately 350 °C, depending on the specific alloy steel). Lower bainite exhibits hardness values similar to tempered martensite and is preferred in applications where elastic characteristics are desired.

Due to their diverse microstructures, micro-alloyed steels are increasingly being adopted across various industries. While extensive literature exists on the processing routes of these steels, experimental data on their suitability for fracture mechanics-based design and manufacturing approaches is relatively scarce, particularly in two areas: (1) the alteration of fundamental fracture mechanics properties of micro-alloyed steels in the presence of structural restraints such as pre-stress and pre-strain, and (2) a comparative study of the effect of heat treatment practices on the fracture mechanics properties of micro-alloyed steels relative to their as-rolled conditions. This study addresses these gaps by experimentally determining the quasi-static initiation fracture toughness (J1c) of low carbon (0.19%) micro-alloyed steel in its as-rolled condition, following ASTM E-1820 standards, without any heat treatment. Additionally, the study examines the effects of normalizing, shot-peening, and cyaniding followed by shot-peening on the fracture toughness parameter. The results indicate that normalizing, shot-peening, and cyaniding, followed by shot-peening, positively influence the initiation fracture toughness of this micro-alloyed steel.