EFFECTS OF Q & P PROCESS PARAMETERS ON PROPERTIES OF 42 SiCr STEEL

The requirement for high strength and good ductility possesses problems in today’s advanced steels. This problem can be tackled by appropriate heat treatment which produces suitable microstructures. By this means, ultimate strengths of about 2000 MPa and elongations of more than 10% can be obtained. One of such advanced heat treatment techniques is the Q&P (Quenching and Partitioning) process. It produces a mixture of martensite and retained austenite, where the latter is an important agent in raising the ductility of steel. In this experiment, a low-alloy steel with 0.41% carbon and manganese, silicon and chromium was used. An air furnace and a salt bath were employed for heat treatment and quenching, respectively. In order to obtain the best ultimate strength and elongation levels, partitioning temperatures of 250°C and 300°C were applied. Partitioning involves carbon diffusion from super-saturated martensite into retained austenite and tempering of hardening microstructure. Effects of the quenching temperatures of 200°C and 150°C were studied as well. To map the impact of the Q&P process on mechanical properties, an additional schedule with conventional quenching and tempering was carried out. Upon optimization of the parameters, the process produced martensite with a small amount of bainite and retained austenite. The ultimate strength was between 1930 and 2080 MPa and the elongation levels were from 9 to 16%.


Introduction
One of modern treatment routes for high-strength low-alloy steels is Q&P processing.It leads to high strength and good ductility because it produces martensitic microstructure with a small amount of retained austenite [1][2][3].Q&P processing involves rapid cooling from a soaking temperature to a quenching temperature (QT), which lies between the Ms and Mf, in order to prevent martensitic transformation from propagation through the entire workpiece [4,5].The subsequent reheating and holding at the partitioning temperature (PT), which is near or just above the Ms, causes excess carbon to migrate from super-saturated martensite into retained austenite which thus remains stable even after cooling to room temperature [6][7][8].Martensite may coarsen in the process.The key stage of this process is carbon redistribution at the partitioning temperature [9][10][11].DOI  The purpose of the Q&P process is to produce very fine martensite with retained austenite between martensite needles.Austenite can also be stabilised by appropriate additions of manganese, silicon and chromium to the steel [12,13].By this means, carbide formation is suppressed and solid solution retains dissolved carbon.Thus, ultimate strengths of more than 2080 MPa along with more than 18% elongation can be obtained.Various kinds of alloying and processing parameters can deliver a broad range of final mechanical properties.For instance, integrating Q&P processing into the hot stamping process for making car body parts opens new opportunities for using high-strength parts in this sector [14,15].

Experimental programme
Q&P processing comprises a number of important parameters which have a profound impact on microstructural evolution and, in turn, on mechanical properties.Among them, the crucial ones are the quenching and partitioning temperatures.

Experimental Material
The experimental programme was carried out on 42SiCr steel.It contains 0.43 wt.% carbon and is alloyed with silicon, chromium and manganese (Table 1).The single most important element with influence on this process is carbon.Carbon stabilises retained austenite, strengthens the solid solution and forms pearlite and carbides.Austenite can also be stabilised by additions of manganese and silicon.Chromium is another element which slows down pearlite and bainite transformations [16,17].The JMatPro software was employed for calculating phase transformation temperatures.Among them, the most important ones are the martensite-start Ms and martensite-finish Mf temperatures.In this steel, the Ms was calculated as 294°C and the Mf as 173°C [18][19][20].The steel was supplied as rolled 10-mm plates with a pearlite-ferrite microstructure and a hardness of 290 HV10 (Fig. 1).The experimental specimens had the size of 17×300 mm and a thickness of 10 mm.Five heat treatment sequences were designed for the experimental programme in order to describe the effects of the quenching and partitioning temperatures.For comparison, conventional quenching and tempering was carried out as well.
Final microstructures were documented using optical and scanning electron microscopy.Mechanical properties were measured by tensile and HV10 hardness testing.The dimensions of the tensile test sample were L0 = 15 mm and S0 = 19.5 mm.

Heat Treatment
The treatment was performed in a furnace with no protective atmosphere.The first step was heating to the austenitizing temperature 950°C/21 min.The second one involved water quenching which was finished so that undershooting of the prescribed quenching temperature was prevented.The third step, final quenching to the quenching temperature, took place in a salt bath.The specimen was then reheated in a furnace and held at the partitioning temperature (Fig. 2).Monitoring of the entire treatment was provided by means of a thermocouple placed inside the specimen.This enabled the cooling process to be stopped at a particular temperature.a scenario where the martensite transformation is interrupted at a lower quenching temperature.There were two partitioning temperatures: 250°C and 300°C.The higher one was to lead to higher tempering, and thus to higher elongation values.Conventional quenching and tempering was carried out as well: soaking 950°C was followed by water quenching to room temperature and reheating in a furnace to 250°C and holding for 10 minutes.
Table 2 Q&P process parameters for the 42SiCr experimental steel and final mechanical properties

Discussions and results
The first sequence was conventional hardening, i.e. quenching and tempering.In this sequence 1, soaking at 950°C/21 min.was followed by water quenching to room temperature and tempering at 250°C/10 min.It produced a microstructure with tempered martensite and bainite (Fig. 3).Fine carbides were present, predominantly in bainite areas.A scanning electron micrograph showed that the material had been highly tempered.This finding was in agreement with the hardness value of 629 HV10, tensile strength of 2060 MPa and elongation of 11%.Subsequently, the Q&P process was carried out (sequences 2 to 5).Sequence 2 comprised heating to the austenitizing temperature, water quenching to approx.480°C and transfer of the specimen into a salt bath at the quenching temperature of 200°C.After temperature homogenization, the specimen was placed into a furnace at 250°C.After temperature homogenization, 10-minute partitioning was performed, followed by cooling in air.This procedure led to a microstructure of martensite with bainite and retained austenite which had fewer fine carbide precipitates (Fig. 4).Hardness was 588 HV10.In the quenched and tempered specimen, it was higher, 629 HV10.After this sequence, the ultimate strength was lower than after sequence 1: 1995 MPa.By contrast, elongation was higher: 15%.In sequence 3, holding at the partitioning temperature was extended from 10 to 20 minutes, which could have led to higher elongation levels.Again, the resulting microstructure consisted of martensite, bainite and retained austenite.Extending the time at the temperature Tp had a visible effect on the microstructure.The martensite was more highly-tempered and the surface relief was more prominent.(Fig. 5).Mechanical properties remained almost unchanged.Hence, the extended time at temperature had no effect.The ultimate strength, elongation and hardness were 1990 MPa, 15% and 575 HV10, respectively.Sequence 4 comprised a partitioning temperature of 300°C which was higher than that in the other sequences: 250°C.The remaining parameters were identical: austenitizing temperature and time and subsequent quenching in two coolants.Partitioning took place at 300°C for 10 minutes and was followed by cooling to room temperature.The result was martensite with retained austenite.Higher partitioning temperature has not led to stronger precipitation of carbides or to their coarsening.Martensite was more highly-tempered and the surface relief was more prominent, as in the previous case.(Fig. 6).The increased partitioning temperature had no appreciable impact on the elongation value.It was 16%.The ultimate strength decreased to 1840 MPa and hardness decreased slightly to 565 HV10.
In the last sequence, the fifth one, the temperature of quenching in the salt bath was lowered from 200°C to 150°C.Partitioning took place in a furnace at 250°C for 10 minutes, followed by air cooling.Neither this sequence has led to any appreciable change in the nature and morphology of the microstructure.Martensite was fine and contained fewer carbides than in the previous cases.This sequence has produced the highest ultimate strength: 2080 MPa, along with an elongation of 14% and a high hardness of 603 HV10 (Fig. 7).Fig. 7 Sequence 5: quenching temperature 150°C -partitioning temperature 250°C, time at temperature 10 mins, martensite with bainite and retained austenite, scanning electron micrograph Two sequences, 1 and 5, were selected for a more detailed overview of results of experiments.After tensile test was completed, the specimens from these sequences were examined using electron microscopy.Fracture surfaces were observed in both specimens.Although the microstructures produced by both sequences were martensitic and had a relatively high hardness, ductile fractures with dimples were found in both specimens (Figs. 8, 9).This finding confirms that the material had good elongation values up to 15%.

Conclusion
Modern heat treatment methods for high-strength steels deliver remarkable mechanical properties, and therefore open new opportunities for their use.This article discusses Q&P processing of low-alloy 42SiCr steel.Five heat treatment sequences were tested.Various quenching and partitioning temperatures were used.A sequence with conventional quenching and tempering was carried out for comparison.The objective was to find such parameters of the Q&P process which lead to strengths of about 2000 MPa while maintaining elongation of approx.15% in the material.In all cases, martensitic microstructures were obtained with certain fractions of bainite and retained austenite, various configurations of carbide precipitates and amount of tempering.It was found that conventional quenching and tempering can lead to ultimate strengths that exceed 2000 MPa along with elongation of 11% in this steel.In the Q&P process, the quenching temperature proved to be crucial.By reducing the quenching temperature from 200°C to 150°C, the highest ultimate strength was obtained: 2080 MPa, combined with an elongation level of 14%.

Fig. 1
Fig. 1 Scanning electron micrograph of the as-received microstructure of 42SiCr steel

Fig. 2
Fig. 2 Schematic diagram of heat treatment sequences