The transformation-induced plasticity (TRIP) in advanced high-strength steels (AHSS) is reviewed, where the main concepts and the recent progress in the processing and properties of AHSS are introduced. The metastable austenitic stainless and multiphase TRIP-assisted steels, as well as the more recent third generation AHSS grades, namely the medium-Mn and quenching and partitioning (Q&P) steels, are critically discussed. These steels utilize the TRIP effect and the enhanced work-hardening rate through the transformation of (retained) austenite in their microstructures to martensite during plastic deformation for the improvement of strength-ductility balance, which make them especially suitable for the automotive industry to be used in the lightweight car body for addressing the safety, fuel consumption, and air pollution issues. The kinetics of strain-induced martensitic transformation (SIMT) as well as the effects of chemical composition, grain size, deformation temperature, strain rate, and deformation mode on the austenite stability are reviewed. The effects of holding temperature and time during the isothermal bainitic transformation (IBT) in TRIP-aided steels, during the austenite-reverted-transformation (ART) annealing in medium-Mn steels, and during the quenching and partitioning steps in the Q&P steels are critically discussed towards enhancement of the amount of retained austenite and optimization of strength-ductility trade off. The alternative thermomechanical processing routes as well as the modified grades such as δ-TRIP and quenching-partitioning-tempering steels are also introduced.

Transformation-induced plasticity (TRIP) in advanced steels: A review

Deformation-induced martensite in austenitic stainless steels: A review

Thermal mechanisms of microstructural refinement in steels were reviewed. These include thermal cycling, martensite process, and static recrystallization (SRX), in which the dominant stage of microstructural refinement is governed by an annealing treatment of a deformed or an undeformed material. Recent progress in grain refinement by thermal cycling for the body-centered cubic, face-centered cubic, and dual phase (DP) steels was introduced. The application of the cold rolling and subsequent annealing of a martensite starting structure (martensite process) for grain refinement of low-carbon and DP steels was reviewed. The formation and reversion of strain-induced martensite in metastable austenitic stainless steels and their effects on the microstructural evolutions were critically discussed. Moreover, the repetition of the martensite process and its limitations were explained. Important findings on the SRX of ferrite and austenite for grain refinement as well as the recrystallization kinetics were presented. Finally, the concepts of controlled rolling for grain refinement and the interaction of austenite recrystallization and strain-induced precipitation in microalloyed steels during thermomechanical processing were also reviewed. This short overview presents the opportunities that the conventional and innovative processing routes can offer for grain refinement of steels.

Thermal Mechanisms of Grain Refinement in Steels: A Review

Heavy metals in high concentrations can cause health and environmental damage. Nanosilver is an emerging heavy metal which has a bright future of use in many applications. Here we report on the levels of silver and other heavy metals in municipal sewage sludge. Five towns in Limpopo province of South Africa were selected and the sludge from their wastewater treatment plants (WWTPs) was collected and analysed. The acid digested sewage sludge samples were analysed using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) methods. The concentrations of silver found were low, but significant, in the range 0.22 to 21.93 mg/kg dry mass. The highest concentration of silver was found in Louis Trichardt town with a concentration of 21.93 ± 0.38 mg/kg dry mass while the lowest was Thohoyandou with a concentration of 6.13 ± 0.12 mg/kg dry mass. A control sludge sample from a pit latrine had trace levels of silver at 0.22 ± 0.01 mg/kg dry mass. The result showed that silver was indeed present in the wastewater sewage sludge and at present there is no DWAF guideline standard. The average Cd concentration was 3.10 mg/kg dry mass for Polokwane municipality. Polokwane and Louis Trichardt municipalities exhibited high levels of Pb, in excess DWAF guidelines, in sludge at 102.83 and 171.87 mg/kg respectfully. In all the WWTPs the zinc and copper concentrations were in excess of DWAF guidelines. The presence of heavy metals in the sewage sludge in excess of DWAF guidelines presents environmental hazards should the sludge be applied as a soil ameliorant.

https://www.mdpi.com/1660-4601/11/3/2569/pdf

Assessment of Heavy Metals in Municipal Sewage Sludge: A Case Study of Limpopo Province, South Africa

Strength properties of annealed austenitic stainless steels are relatively low and therefore improvements are desired for constructional applications. The reversion of deformation induced martensite to fine-grained austenite has been found to be an efficient method to increase significantly the yield strength of metastable austenitic stainless steels without impairing much their ductility. Research has been conducted during thirty years in many research groups so that the features of the reversion process and enhanced properties are reported in numerous papers. This review covers the main variables and phenomena during the reversion processing and lists the static and dynamic mechanical properties obtained in laboratory experiments, highlighting them to exceed those of temper rolled sheets. Moreover, formability, weldability and corrosion resistant aspects are discussed and finally the advantage of refined grain structure for medical applications is stated. The reversion process has been utilized industrially in a very limited extent, but apparently, it could provide a feasible processing route for strengthened austenitic stainless steels.

https://www.mdpi.com/2075-4701/10/2/281/pdf

Processing and Properties of Reversion-Treated Austenitic Stainless Steels

This paper presents a comprehensive study on the strain-induced martensitic transformation and reversion transformation of the strain-induced martensite in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis. Tensile deformation was applied at temperatures between room temperature and 213 K (−60 °C) in order to obtain a different volume fraction of strain-induced martensite (up to ~70 pct). The volume fraction of the strain-induced martensite, measured by the magnetometric method, was correlated with the total elongation, hardness, and linear thermal expansion coefficient. The thermal expansion coefficient, as well as the hardness of the strain-induced martensitic phase was evaluated. The in-situ thermal treatment experiments showed unusual changes in the kinetics of the reverse transformation (α′ → γ). The X-ray diffraction analysis revealed that the reverse transformation may be stress assisted—strains inherited from the martensitic transformation may increase its kinetics at the lower annealing temperature range. More importantly, the transmission Kikuchi diffraction measurements showed that the reverse transformation of the strain-induced martensite proceeds through a displacive, diffusionless mechanism, maintaining the Kurdjumov–Sachs crystallographic relationship between the martensite and the reverted austenite. This finding is in contradiction to the results reported by other researchers for a similar alloy composition.

/pdf/the-investigation-of-strain-induced-martensite-reverse-1bb75lm8z8.pdf

The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel

Purpose: of this work is to present possibility of calculation of pearlite dissolution finish temperature Ac1f during heating of hypoeutoctoid steels. Design/methodology/approach: The presented multiple linear regression equations for calculating the Ac1f temperature are based on experimental data set containing chemical composition and values of critical temperatures obtained by use of the dilatometric technique at the own laboratory only. Findings: The elaborated multiple linear regression equations for calculating the critical temperatures are an alternative to dilatometric examinations to obtain data necessary for proper heat treatment conditions planning. Research limitations/implications: All presented equations for calculating pearlite dissolution finish temperature are limited by range of mass concentrations of elements which is a consequence of limited data set used for elaboration of these equations. The obtained relationships do not concern other factors influencing Ac1f temperature such as heating rate, grain size and interlamellar spacing of pearlite. Practical implications: Broadening the knowledge on the chemical composition influence on the critical temperatures, which will help in designing heat treatment conditions, especially of the Dual Phase steels. Originality/value: An attempt was made to find out a multiple linear regression formula between chemical composition and the pearlite dissolution finish temperature of hypoeutectoid steels.

Critical points of hypoeutectoid steel - prediction of the pearlite dissolution finish temperature Ac 1f

Purpose: of this work is to present possibility of proper determination of pearlite dissolution finish temperature Ac1f during heating of hypoeutoctoid steels Design/methodology/approach: The presented schemes of splitting the first derivative curve of hypoeutectoid steels dilatograms are based on experimental dilatograms and their first derivatives obtained by use of the dilatometric technique Findings: The nine possible schemes of splitting the first derivative curve of hypoeutectoid steels dilatograms were developed These schemes have been developed for three main cases: ferrite to austenite transformation temperature range is wider than for pearlite to austenite transformation, ferrite to austenite transformation temperature range has the same width as for the pearlite to austenite transformation and ferrite to austenite transformation temperature range is shorter than for pearlite to austenite transformation Presented schemes are fully compatible with experimental results Research limitations/implications: Further verification of correctness of the presented schemes and the development of appropriate curve fitting software is needed Practical implications: Broadening the knowledge on the phase transformation kinetics of hypoeutectoid steels during heating The basics for the automate process of determination of critical temperatures in steels, especially the Ac1f temperature, were established Originality/value: The originally schemes of splitting the first derivative curve of hypoeutectoid steels dilatograms were developed

Dilatometric examination of continuously heated austenite formation in hypoeutectoid steels

The paper presents a new approach to interpretation of hypoeutectoid steel dilatograms upon heating between the austenite start and austenite finish temperatures. The proposed derivative curve separation method yields data which accords with the experimental results. Besides the new method appears a useful tool whereby it becomes possible to confirm the evidence reported earlier in literature that when a hypoeutectoid steel heats up to between the A1 and the A3 both ferrite and pearlite start to transform into austenite almost at the same time.

Determination of Critical Points of Hypoeutectoid Steels / Wyznaczanie Punktów Krytycznych W Stalach Podeutektoidalnych

Dilatometry is the most commonly method of producing CCT diagrams and analyzing phase transformations during cooling (as well as transformations during heating) and it permits the real-time monitoring of the extent of reaction in terms of dimensional changes due to phase transformation. All modern dilatometers are fitted with computerised systems which collect dimensional change signals versus temperature to plot a dilatometric curve and also to calculate and plot the derivative of the relative dimensional change with respect to temperature. Unfortunately, elaborated by dilatometers manufacturers software, in many cases could lead to wrong interpretation of phase transformations during cooling.

B. Pawłowski

Papers

The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel

Critical points of hypoeutectoid steel - prediction of the pearlite dissolution finish temperature Ac 1f

Dilatometric examination of continuously heated austenite formation in hypoeutectoid steels

Determination of Critical Points of Hypoeutectoid Steels / Wyznaczanie Punktów Krytycznych W Stalach Podeutektoidalnych

Improper Interpretation of Dilatometric Data for Cooling Transformation in Steels