Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel M. ARBAB REHAN, ANNA MEDVEDEVA, LARS-ERIK SVENSSON, and LEIF KARLSSON Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 °C and 600 °C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 °C) resulted in retained austenite transformation to martensite on cooling. The martensite start (Ms) and martensite finish (Mf) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the Ms temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 °C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temper- ature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed. DOI: 10.1007/s11661-017-4232-5 Ó The Author(s) 2017. This article is an open access publication I. INTRODUCTION transformation of austenite to martensite does not go to completion when quenching to room temperature INDUSTRIAL processes, such as rolling, cutting, after austenitization. As a consequence, some amount of forming, and punching, taking place at temperatures austenite is retained in the as-quenched microstructure, below 473 K (200 °C) are usually referred to as ‘‘cold which otherwise consists of martensite (and sometimes work processes.’’ Steels that are used to make tools for B) and undissolved primary carbides. Before being put these applications are, hence, called ‘‘cold work tool [1,2] into use as a tool, the steel is then tempered to enhance steels.’’ These steels must be able of to attain its ductility and to precipitate alloy carbides. The properties, such as high hardness, high compressive tempering temperature influences both the stability of strength, and good toughness, to be suitable for cold retained austenite and the precipitation of carbides in forming of advanced high-strength steels. the tempered martensite; therefore, it has a large effect The fairly high alloying content of 5 wt pct Cr cold on the properties.[1,3–5] work tool steels results in low martensite start (Ms) Retained austenite transformation has been a subject of and martensite finish (Mf) temperatures. Thus, the scientific interest for a long time, but characterization and understanding is still a challenge.[4–8] Literature suggests that the retained austenite can transform into ferrite and cementite,[6] martensite,[5] or B.[4] Ferrite and cementite M. ARBAB REHAN is with the Department of Engineering form during isothermal treatment at the tempering Science, University West, 461 86, Trollha¨ttan, Sweden, and also with temperature, while the latter transformations occur Uddeholms AB, 683 85, Hagfors, Sweden. Contact email: during cooling from the tempering temperature. It has [email protected] ANNA MEDVEDEVA is with Uddeholms AB, been shown that the isothermal transformation into 683 85, Hagfors, Sweden. LARS-ERIK SVENSSON and LEIF [6–9] KARLSSON are with the Department of Engineering Science, ferrite and carbide is a two-step process. First, University West. precipitation of cementite occurs from retained austenite, Manuscript submitted November 21, 2016. which is gradually followed by transformation into ferrite Article published online September 19, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 48A, NOVEMBER 2017—5233 and cementite.[6] These observations have been reported However, understanding retained austenite transfor- for carbon, bainitic, and hot work tool steels.[10,11] Hence, mation required tempering treatments with variations in the mechanism by which retained austenite transforms both temperatures and holding times. Heat treatments depends on steel composition as well as heat treatment conducted for the study are listed in Table II, where procedure and is of great importance for the resulting tempering at 798 K (525 °C) includes a range of holding mechanical properties.[12] Although the mechanisms are times, the standard tempering consisting of a double known, little information is available in the literature tempering cycle at 798 K (525 °C) (Figure 1) and about the transformation of retained austenite during long-time tempering at 873 K (600 °C). The 798 K (525 tempering of cold work tool steels. °C) heat treatments with increasing holding times were The present investigation was designed to improve the conducted to investigate the stability of retained austen- understanding of retained austenite transformation ite. This was done by examining the correlation between during tempering and cooling of a 5 wt pct Cr cold the changes in the composition of retained austenite work tool steel. With this purpose, tempering temper- during tempering and its transformation to martensite atures of 798 K and 873 K (525 °C and 600 °C) were on cooling. The 873 K (600 °C) treatment with long selected and holding times were varied to understand the holding time was aimed at understanding retained mechanisms involved during the transformation. The austenite transformation during isothermal tempering. transformation behavior during heat treatments was studied with dilatometry, and the resulting microstruc- C. Microstructural Characterization tures were characterized with scanning electron micro- scopy (SEM) and X-ray diffraction (XRD). The results Microscopy was carried out on dilatometry specimens are used to understand the retained austenite transfor- by SEM (FEI Quanta 600F). Sample preparation mation when the steel is tempered following the stan- involved grinding of the specimen with abrasive papers dard heat treatment. of mesh size 180, 300, 500, and 1200. The surfaces were polished first to 3 lm and then 1 lm with diamond suspension. The as-quenched microstructure was II. EXPERIMENTAL revealed by etching with 2 pct Nital for 1 minute, while tempered microstructures were etched first in Picral for 2 A. Material seconds and then in 2 pct Nital for 5 seconds. The An electroslag remelted ingot with the chemical microstructure constituents bainite, fresh martensite, composition of Uddeholm Caldie was produced. This tempered martensite, and retained austenite are referred was hot rolled into a round billet with a radius of 80 mm to with the acronyms B, FM, TM, and RA, respectively, and then soft annealed. The chemical composition from going forward. the center of the steel billet is presented in Table I. XRD was conducted with Seifert 3003 equipment to measure the volume percent and the lattice parameter of RA. XRD was carried out with unfiltered Cr Ka B. Heat Treatment radiation at a voltage of 40 kV and a step size of 0.1 All specimens were heat treated in a push rod dilatome- deg over a 2h range of 50 to 165 deg. The software ter (Dil 805 A/D, Bahr-Thermoanalyse). Cylindrical Rayflex 2.408 (GE & Inspection Technologies) was used specimens (l 9 d =109 4 mm) in soft-annealed condition were taken from the center of the steel billet. The specimens were heated with a high-frequency inductive 1323 K (1050°C) coil in a vacuum chamber. Specimen temperatures were measured by a thermocouple, spot welded on the longi- 0.5 h tudinal surface of the specimen. The heat treatments 3.5 K/s consisted of hardening and tempering. All specimens were 1 K/s heated to 1323 K (1050 °C) with a rate of 3.5 K/s, held for 798 K (525°C) 798 K (525°C) 30 minutes, followed by cooling to room temperature. Helium gas was used as a quenching medium to achieve a Temperature (°C) 2 h 2 h cooling time of 300 seconds between 1073 K and 773 K 1 K/s 1 K/s (800 °C and 500 °C). Heating to the tempering temper- ature was done at a rate of 17 K/s. In the standard tempering treatment, the temperature was held for 2 Time (h) hours before cooling to room temperature. This cycle was repeated two times (Figure 1). Fig. 1—Schematic diagram illustrating the standard heat treatment cycles of the investigated steel. Table I. Chemical Composition of the Investigated Steel (Weight Percent) CSiMnCrMoV 0.7 0.2 0.5 5.0 2.2 0.5 5234—VOLUME 48A, NOVEMBER 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A to measure the relative intensities of the diffracted peaks using Uddeholms internal data base ‘‘Tooling 11.’’ The of martensite (110)a, (200)a, and (211)a and the motivation was to predict the composition of the austen- diffracted peaks of RA (111)c, (200)c, and (220)c. Use ite after austenitization to permit a comparison with the of these numbers of peaks avoids possible bias due to carbon content of RA after tempering treatments. crystallographic texture.[13] The curve fitting of the diffracted peaks was performed on the plot of intensity as a function of 2h. The volume percent of RA was III. RESULTS determined by comparing the integrated intensities of martensite and RA according to Cullity.[14] The uncer- A. Thermodynamic Calculations tainty of measurements was estimated to ±2 vol pct. The thermodynamic calculations showed two phases Retained austenite contents of less than 2 vol pct, in equilibrium at 1323 K (1050 °C), i.e., austenite and therefore, cannot be measured reliably by XRD.