Optimization of Q-P process parameters with regard to final microstructures and properties.

Kucerova Ludmila ; Aisman, David ; Jirkova, Hana 等

1. INTRODUCTION

Modern concepts of steel treatment show that the potential of this traditional material is far from being spent. New types of low-alloyed high strength steels with good ductility have been developed in recent years due to the application of innovative methods of heat and thermo-mechanical treatment. While classical heat treatment processes have been usually able to increase either strength or ductility, these new methods like TRIP (transformation induced plasticity) steel processing, long-term baintic annealing or Q-P (Quenching & Partitioning) process enable us to obtain microstructures with excellent strength and relatively high ductility (Edmonds et al., 2006).

Q-P process results in the microstructure of cubic martensite with stabilized retained austenite, which originates from special heat treatment cycles (Santofimia et al., 2008). The steel is first austenized, then quenched at the temperature between Ms and Mf and finally annealed at slightly higher temperature (Fig.1). The partitioning of carbon occurred during the hold at annealing temperature, when carbon diffuses from supersaturated tetragonal martensite into remaining untransformed austenite. Higher carbon content leads to better stability of final retained austenite during the cooling to room temperature. Carbide precipitation during partitioning annealing is therefore undesirable and should be avoided (Nayak et al., 2008).

2. EXPERIMENTAL PROGRAMM

42SiCr steel with about 0,4%C was chosen as an experimental material. The main alloying element of this steel was silicon, which should suppress carbide precipitation during partitioning annealing. Silicon content was kept around 2%. The steel was further alloyed by lower amount of manganese to stabilize austenite and reduce pearlitic transformation and also by over 1% of chromium, which should increase the strength of the steel. The initial microstructure was pearlitic-ferritic with 14,5% of ferrite and it had the ultimate strength of 981MPa with the ductility A5mm of 30%. The size of used specimens was in all cases the same: 55x18x25 mm.

Q-P process parameters were optimized in several steps. First of all, austenization temperature 900[degrees]C was determined according to the dilatometric measurements of phase transformation temperatures and also according to a phenomenological model (Hauserova, 2008). Annealing temperature and hold were tested in the next step. Several specimens with the same austenization temperature were prepared and cooled either directly into salt bath with the temperature of 250[degrees]C or they were firstly cooled in water for 2s and subsequently moved to the salt bath with the temperature of 250[degrees]C. The hold in salt bath varied from 5 to 20 minutes. The temperature of the specimens was continuously monitored by attached thermocouples. Resulting microstructures were predominately martensitic with 5-10% of retained austenite, ultimate tensile strength over 2000MPa and ductility [A.sub.5mm] of 14-18% (Aisman et al., 2008). Two specimens were furthermore cooled from austenization temperature in water and oil, directly to the room temperature to determine comparable properties of this steel after common quenching. Ductility [A.sub.5mm] of quenched specimens was predictable low, around 2%, while the ultimate tensile strength reached 2250MPa.

Using the results of previously mentioned experiments the parameters of the overall Q-P process were tested in the next step. Six processing strategies were designed, all having the same austenization conditions 900[degrees]C / 25min (Tab.1), but different undercooling and annealing temperatures (Fig.1). Strategies 1-4 tested several undercooling temperatures and the effect of two different partitioning temperatures.

Processing strategies 1-4 therefore included undercooling in cold water at the temperatures between 110[degrees]C and 200[degrees]C and subsequent movement for 10 min into salt bath with the temperature of 250[degrees]C or 300[degrees]C. Processing strategies 5-6 on the other hand involved cooling from austenization temperature into salt bath or hot water to the temperature of 150[degrees]C. This enabled us to compare the influence of different cooling rates on resulting microstructures and properties. Undercooled specimens were then moved for 10 min into a furnace with the temperature of 250[degrees]C.

[FIGURE 1 OMITTED]

3. RESULTS

Microstructures after heat treatment were observed by the means of light microscopy and laser scanning confocal microscopy. Volume fractions of retained austenite were obtained using X-ray diffraction phase analysis. The microstructures were in all cases predominantly martensitic with 8-14% of retained austenite (Fig.2, Fig.3).

Yield and ultimate strength and elongation [A.sub.5mm] were measured by tensile test for processing strategies 1-4 and the hardness HV 10 of microstructures after each processing was determined (Tab.1). Ultimate tensile strength of specimens after Q-P process was slightly under 2000MPa, the maximum reaching 1965MPa for processing strategies 1 and 2. The ductility [A.sub.5mm] after these two processing was increased up to 20%. The biggest drop of ultimate tensile strength to the 1770MPa occurred after processing 4, which combined relatively high undercooling temperature of 200[degrees]C with higher annealing temperature of 300[degrees]C. The microstructure after this processing also possessed the highest ductility [A.sub.5mm] of 22%, however this ductility increment was negligible with respect to ductilities of microstructures after processing 1 and 2. Hardness values followed the trend of ultimate tensile strengths. The hardness after processing 5, which involved undercooling in salt bath, was lower than hardness after processing 2 and 6, which had the same undercooling temperature, but higher cooling rates.

4. CONCLUSION

Several strategies of Q-P processing heat treatment were tested. Resulting microstructures consisted of martensite and retained austenite. Higher fractions of austenite found in microstructures after Q-P processing corresponded to higher ductility and lower hardness of these microstructures in comparison to directly quenched specimens. Mechanical properties of steel after different Q-P processing strategies were convenient.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The most promising mechanical properties were obtained after heat treatment consisting of: austenization 900[degrees]C/20min followed by undercooling in cold water to 110-150[degrees]C and partitioning hold 250[degrees]C / 10min Ultimate strength was slightly under 2000MPa with the ductility [A.sub.5mm] of 20%. Higher undercooling temperature of 200[degrees]C resulted in 100 MPa decrease of strength. Higher partitioning temperature of 300[degrees]C led to an even higher drop of ultimate strength which might be caused either by higher tempering of martensite or by carbide precipitation.

The potential of Q-P process however has not yet been totally investigated and there are still several parameters to test, particularly partitioning temperature and hold and the rate of undercooling should be analyzed in more detail.

5. ACKNOWLEDGEMENTS

This paper includes results obtained within the project 1M06032 Research Centre of Forming Technology.

6. REFERENCES

Aisman, D., Jirkova, H., Skalova, L. & Masek, B. (2008). Testing the Influence of the Q-P Process on the Development in High Strength Low-alloyed Steels, Proceedings of the 19th International DAAAM Symposium, pp.007-008, KATALINIC Branko, ISBN 978-3-901509 68-1, Trnava, 10 2008

Edmonds, D.V., K, Rizzo, F.C., De Cooman, B.C., D.K. Matlock & Speer, J.G. (2006). Quenching and partitioning martensite-A novel steel heat treatment, Materials Science and Engineering A, Vol. 438-440, (2006), pp 25-34, ISSN: 0921-5093

Hausserova, D. (2008). Transformation behaviour of high strength steel, Diploma thesis, Universtiy of West Bohemia in Pilsen, 2008, Pilsen

Nayak, S.S., Anumolu, R., Misra R.D.K., Kim K.H. & Lee D.L (2008). Microstructure-hardness relationship in quenched and partitioned medium-carbon and high-carbon steels containing silicon. Materials Science and Engineering A, Vol. 498 (2008) 442-156, ISSN: 0921-5093

Santofimia, M.J., Zhao, L., Petrov, R. & Sietsma, J.(2008). Characterization of the microstructure obtained by the quenching and partitioning process in a low-carbon steel, Materials Characterization, Vol. 59, (2008), pp. 17581764, ISSN: 1044-5803

Tab. 1. Processing parameters and hardness HV10

Strategy Undercooling Partitioning annealing HV10
 [[degrees]C]/medium [[degrees]C]/min/medium

1 110 / water 250 / 10 / salt bath 570
2 150 / water 250 / 10 / salt bath 569
3 200 / water 250 / 10 / salt bath 560
4 200 / water 300 / 10 / salt bath 524
5 150 / slat bath 250 / 10 / furnace 531
6 150 / hot water 250 / 10 / furnace 563