ASTM A694 F60 – HEAT TREATMENT AND MECHANICAL PROPERTIES

Martin BALCAR, Jaroslav NOVÁK, Libor SOCHOR, Pavel FILA, Ludvík MARTÍNEK

ŽĎAS, a.s., Strojírenská 6,

591 71 Žďár nad Sázavou, Czech Republic,

Tel.:+420 566 64 2136, Fax.: +420 566 64 2831, Email: martin.balcar@zdas.cz

Abstract

Production of heavy steel forgings of microalloy steels seeks possibilities to take advantages associated with the benefit of application of microalloying elements and thermomechanical treatment at the level of mechanical properties attained, known from production of sheets, strips and tubes.

The paper documents the influence of quenching temperature on values of mechanical properties and structure of F60 steel according to ASTM A694. Verification of quenching temperature influence contributes to optimization and determination of a complex method of microalloy steel heat treatment.

Steel structure and mechanical properties after the quenching constitute initial as well as basic criterion to achieve requested mechanical properties at properly chosen tempering temperature. Keywords: A694 F60, HSLA steel, Quenching and Tempering, Mechanical Properties

1. INTRODUCTION

As the requirements for structural steel properties are growing, the development of use of microalloying elements even in the field of production of forgings and castings takes place.

Production of heavy steel forgings of microalloy steels does not allow to immediately take advantages associated with the benefit of application of microalloying elements and thermomechanical treatment known from production of sheets, strips and tubes. In the case of production of steel forgings, the forming and heat treatment process mainly features different forming and heat treatment conditions compared to thin-walled products (sheets, strips, tubes).

Development and verification of F60 steel production and treatment technology according to ASTM A694 under ZDAS, Inc. conditions constitute a

number of technological changes and introduction of new process elements in the field of steel making and subsequent thermomechanical treatment.

Verification of quenching temperature influence on properties and structure of F60 forged steel contributes to optimization of complex HSLA steel making technology at ZDAS, a.s.

2. EXPERIMENTAL MATERIAL

Verification of quenching temperature influence on structure and mechanical properties of modified F60 steel according to ASTM A694 made by EOP/LF/VD technology was carried out on forged piece samples with dimensions (100x100x150)mm. Basic chemical composition of the steel is shown in Table 1.

Table 1: Basic chemical composition HSLA steel F60 (wt. %)

Modified F60 steel according to ASTM A694 is typical low carbon steel with addition of alloying elements, manganese, silicon, nickel and molybdenum. Moreover, the steel is microalloyed by vanadium, aluminium and niobium. Concentration of other elements is at the level like residual elements and the rest is iron [1], [2].

After the forming process, the forgings were “anti-flake” annealed up to a temperature of 650°C for a period of 10 hours and subsequently normalized at a temperature of 930°C with air cooling.

3. LABORATORY HEAT TREATMENT

The heat treatment itself was carried out on forged steel samples under laboratory conditions.

Verification of influence of austenitization – quenching temperature (TA) on the structure and mechanical properties was proposed for the temperature range (880 to 940)°C with subsequent water quenching and tempering TT = 620°C with air cooling. Sample marking and heat treatment carried out are as follows:

L1: TA = 880°C/6 hrs/Water + TT = 620°C/8 hrs/Air L2: TA = 890°C/6 hrs/Water + TT = 620°C/8 hrs/Air L3: TA = 900°C/6 hrs/Water + TT = 620°C/8 hrs/Air L4: TA = 910°C/6 hrs/Water + TT = 620°C/8 hrs/Air L5: TA = 920°C/6 hrs/Water + TT = 620°C/8 hrs/Air L6: TA = 930°C/6 hrs/Water + TT = 620°C/8 hrs/Air L7: TA = 940°C/6 hrs/Water + TT = 620°C/8 hrs/Air

Figure 1: Forged sample (100x100x150)mm

4. MECHANICAL PROPERTIES OF HSLA STEEL ASTM A694 F60

The samples to determine attained parameters of mechanical properties and to evaluate the structure were taken from central zones of the forgings. Evaluation of mechanical properties was carried out in longitudinal direction. Table 2 shows the requested level and attained values of mechanical properties of individual F60 steel samples.

Table 2: Mechanical properties HSLA steel F60 – acc. quenching temperature

Austenitization temperature influence on change in mechanical properties of forged, quenched and tempered F60 steel is visible from Table 2. It is obvious that steel strength increase accompanied by significant toughness drop takes place as the austenitization temperature is increasing.

5. MICROSTRUSCTURE OF HSLA STEEL ASTM A694 F60

Similarly as in the case of mechanical properties evaluation, steel structure evaluation was carried out in the given point of the sample.

Steel structure for concrete heat treatment states (TA = 880, 900, 920, 940°C) is shown below:

After the quenching and tempering, the microstructure of all samples – forgings is practically the same. Ferrite, bainite, granular pearlite and sorbite occur. It is evident from a series of snaps magnified 500 times, where the secondary grain can be compared better, that the secondary grain does not change notably as the quenching temperature is increasing.

Above mentioned fact is confirmed by results of evaluation of austenitic grain size by means oxidation method according to ASTM E 112 – 97 with using the LECO IA32 image analysis. Results of austenitic grain size measurement are shown in Table 3.

Table 3: Grain size - HSLA steel F60 - ASTM E 112 – LECO IA32

From the results shown in Table 3, it is not possible to observe direct influence of quenching temperature on austenitic grain size change. All the samples show a very fine structure [3], [4], [5].

6. CONCLUSIONS

From results of experimental work attained follows direct influence of quenching temperature on mechanical properties of ASTM A694 F60 steel. Was observed growth in strength values and drop in impact value as the temperature of quenching was increasing. The most favorable results of mechanical properties were attained with quenching temperatures 880°C, 890°C and 900°C.

Steel microstructure after the quenching and tempering is comparable for all the samples. Ferrite, bainite, granular pearlite and sorbite occur. Evaluation of austenitic grain size by oxidation method according to ASTM E 112-97 confirmed the grain size uniformity, when comparing all experimental samples, without provable influence of quenching temperature.

Further optimization of steel mechanization properties and structure can be awaited after the verification of influence of the tempering temperature. Subsequently, it will be possible to determine a complex optimized heat treatment process for HSLA steel ASTM A694 F60.

ACKNOWLEDGMENTS The paper has been solved within the EUREKA of the OE 08009 E!4092 MICROST project. The project was implemented based on government resources with financial support of the Ministry of Education, Youth and Sport of the Czech Republic

REFERENCES

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http://www.cbmm.com.br/portug/sources/techlib/info/charact/charact.htm

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[3] Balcar, M., Novák, J., Sochor, L., Fila, P., Martínek, L.: ASTM A694 F60 Heat treatment and mechanical properties. In: 20. mezinárodní konference metalurgie a materiálů METAL 2011. 18. – 20. května 2011. Brno, Hotel Voroněž, TANGER s.r.o. Ostrava. CD-ROM. ISBN 978-80-87297-22-2

[4] Balcar, M., Havelková, L.: Vliv stopových prvků na houževnatost oceli. In: Teorie a praxe výroby a zpracování oceli. Rožnov pod Radhoštěm, TANGER, s.r.o., Ostrava, 2009, p. 10 – 15. ISBN 978-80-87294-01-7

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