Materials and Manufacturing Processes, 22: 474–480, 2007 Copyright © Taylor & Francis Group, LLC ISSN: 1042-6914 print/1532-2475 online DOI: 10.1080/10426910701235934
Effect of Deep Cryogenic Treatment on the Carbide Precipitation and Tribological Behavior of D2 Steel D. Das1 , A. K. Dutta2 , V. Toppo3 , and K. K. Ray4 2
1 Department of Metallurgy and Materials Engineering and Department of Mechanical Engineering, Bengal Engineering and Science University, Shibpur, Howrah, India 3 National Institute of Foundry and Forge Technology, Ranchi, Jharkhand, India 4 Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, India
The influence of deep cryogenic processing in between quenching and tempering (QCT) on the carbide precipitation and the tribological behavior of a commercial AISI D2 steel has been examined. The developed microstructures have been characterized with an emphasis to understand the influence of QCT vis-à-vis conventional quenching and tempering (QT) on the nature, size, morphology, and distribution of carbide particles. The mechanical properties such as hardness and wear resistance of the samples treated by QT and QCT have been evaluated employing Vickers indentation and sliding wear techniques, respectively. It has been demonstrated that deep cryogenic treatment leads to considerable microstructural changes which result in enhanced tribological properties. Keywords AISI D2 steel; Carbide precipitation; Cryogenic treatment; Cryotreatment; Deep cryogenic treatment; Hardening; Martensite; Microstructure; Primary carbide; Retained austenite; Secondary carbide; Tempering; Tribological behavior; Wear resistance; Wear rate.
cannot be directly related to the processing by deep cryogenic treatment, but is related to suitable combination of the resulting fracture toughness and hardness of the steel. Barron [10] examined the effect of cryogenic treatment on different types of steels and reported improvement in wear resistance for cryotreated D2 steel over their conventionally treated counterparts by 8.2 times. This dramatic improvement in wear resistance has been attributed to complete transformation of austenite to martensite by the author. Paulin [11] extended support to Baron’s hypothesis. Based on some earlier reported results and some generated ones on the cryogenic treatment of tool steels, Collins [2] suggested two different mechanisms to be active in cryotreatment: (i) transformation of retained austenite to martensite and (ii) low temperature conditioning of martensite. In addition, Collins [2] also suggested that these two phenomena have different effects on the resultant mechanical properties of the steels. Collins and Dormer [8] later reiterated the same view using extended experimental investigations. Low-temperature conditioning of martensitic structure implies crystallographic and microstructural changes which, on reheating, result in the precipitation of a finer distribution of carbides in the tempered microstructure with consequent increase in toughness as well as in wear resistance. Bensely et al. [5] reported 372% increment of wear resistance of deep cryogenically treated samples of case carburized En 353 steel over the conventionally heat-treated ones. The observed improvement in wear resistance has been attributed to the conversion of retained austenite to martensite in association with finer distribution of precipitated carbides. The effect of deep cryogenic treatment on the improvement of wear resistance of carburized steels was also reported by Preciado et al. [7]. These investigators attributed the improvement to the segregation of carbon atoms and alloying elements during the cryogenic cycle,
Introduction Effect of deep cryogenic treatment on performance of steel, particularly with respect to wear resistance, is the subject matter of quite a few research endeavors over the last few decades [1–3]. “Deep cryogenic treatment” or “cryogenic treatment” or simply “cryotreatment” is different from the conventional “cold treatment”, as the former is carried out at about the liquid nitrogen temperature (77 K) whereas the latter is done at about dry ice temperature (193 K) [2–5]. It has been claimed by several researchers that cryogenic treatment enhances wear resistance of certain steels [1–9]. But the reported magnitudes of the enhancement in wear resistance and the proposed governing mechanisms for such enhancement do not provide any unified picture. Molinari et al. [3] examined the effect of deep cryogenic process on quenched and tempered AISI M2 and H13 steels and reported higher wear resistance for these steels compared to their conventionally treated structures. The cause of increased wear resistance, however, was explained in different manner for the two steels. While for AISI M2 steel the increase in wear resistance was attributed to increased hardness and improved homogeneity in hardness, for AISI H13 steel the increased wear resistance was explained by its increased toughness due to cryotreatment. Leskovsek et al. [1] reported that wear resistance of vacuum heat-treated high-speed steel, when subjected to cryogenic treatment, gets improved by an order of magnitude. These investigators mentioned that the observed beneficial effect Received February 10, 2006; Accepted September 30, 2006 Address correspondence to A. K. Dutta, Department of Mechanical Engineering, Bengal Engineering and Science University, Shibpur, B. Garden, Howrah 711103, India; Fax: +91(33)26684564/2916; E-mail:
[email protected],
[email protected]
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in addition to the transformation of the retained austenite to strain induced martensite. Mohanlal et al. [4] studied the effect of cryogenic treatment on different steels, and by machining tests, these authors obtained nearly 110%, 87%, and 48% increment in tool life for cryotreated T1, M2, and D3 tools, respectively, in comparison to the corresponding standard heat-treated tools. It was postulated by these investigators that the mechanism causing improvement in wear resistance/tool life is by martensitic transformation or precipitation of fine alloying carbides. Yun et al. [9] investigated the influence of deep cryogenic treatment on the microstructures, mechanical properties, and service life of high speed steel tools. These investigators concluded that deep cryogenic treatment transforms retained austenite to martensite; while martensite gets decomposed, ultrafine carbides precipitate out. The latter phenomena has been pointed out as the key factor in increasing the red hardness, strength, and toughness of high-speed steels resulting into 100% increase in the service life of disk mill cutter tools [9]. Huang et al. [12] observed that cryogenic treatment not only facilitates carbide formation, increase in the carbide population and increase in its volume fraction in the martensitic matrix of M2 steel, but it also makes the carbide distribution more homogeneous. By studying the effect of cryogenic treatment on Fe1.4Cr-1C bearing steel, Meng et al. [13] inferred that the improved wear resistance of this steel by cryotreatment was due to favorable distribution of carbide rather than by transformation of retained austenite to martensite. Meng et al. [6] further reported that cryogenic treatment improves the wear resistance of Fe-12Cr-Mo-V-1.4C tool steel by about 110% to 600%, depending on the sliding speed of wear tests. It was envisaged by these authors that cryogenic treatment improves preferential precipitation of fine -carbides during the primary stage of tempering in a high-carbon alloy steel. These carbides might enhance the strength and toughness of the martensite matrix and thus improve the wear resistance. It was proposed by these authors that as a result of cryogenic treatment, iron or substitutional atoms expand and contract, and carbon atoms shift slightly due to lattice deformation promoting formation of -carbide. Available results in the literature pertaining to structureproperty relations of steels subjected to cryogenic treatment are not coherent and the underlying postulated mechanisms for achieving improved mechanical properties like wear resistance are not well crystallized. Most researchers believe that deep cryogenic treatment promotes complete transformation of retained austenite into martensite, and this can be attributed to the enhanced wear resistance of the steels [2, 10], whereas another school claims that the cause of increased wear resistance is the formation of fine carbides in martensite matrix and their uniform distribution [6, 8, 9, 12]. The extent of benefits of these emerging processing routes can only be suitably exploited if the underlying mechanisms of these processes are carefully unfolded in an organized manner. This investigation aims to examine the effects of cryogenic treatment on an AISI grade D2 steel with respect to (a) the nature, size, morphology, and distribution of carbide particles in the microstructure
475 and (b) the resultant mechanical properties like hardness and wear resistance. Experimental A commercially available forged AISI D2 steel bar with composition as shown in the parentheses (1.49 C, 0.29 Mn, 0.42 Si, 11.38 Cr, 0.80 Mo, 0.68 V, 0.028 S, 0.029 P, and Fe-balance, all in wt.%) was used in this investigation. Steel blanks having the approximate size of 24 × 16 × 85 mm, were subjected to conventional (QT) and cryogenic treatments (QCT) in separate batches; the latter being carried out for two different isothermal holding times of 36 and 84 h at 77 K. The QT treatment for the AISI D2 steel consists of hardening and tempering as suggested in the Heat Treater’s Guide [14], whereas in QCT, an additional step of deep cryogenic processing has been incorporated as an intermediate step between hardening and tempering [Fig. 1(a)]. For hardening, the samples were first preheated at the rate of 9 K/min up to 1088 K, held there for 30 min for temperature homogenization and were then austenitized at 1297 K for 30 min. The samples were then quenched to 813 K in a salt bath furnace and were held for 15 min, subsequent to which these were air cooled to room
Figure 1.—(a) Schematic presentation of heat-treatment schedule, and (b) typical deep cryogenic processing cycle.
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D. DAS ET AL. Table 1.—Description of sample codes with heat treatment cycles.
Sample codes
QT QCT36 QCT84
Description of heat-treatment cycles
Hardening + Tempering Hardening + Deep Cryogenic processing for 36 h + Tempering Hardening + Deep Cryogenic processing for 84 h + Tempering
temperature [Fig. 1(a)]. Tempering was carried out at 483 K for 120 min in a salt bath. The deep cryogenic processing was performed in a laboratory made manually controlled cryogenic processor using liquid nitrogen as the medium. The QCT treatment consisted of (a) subjecting the hardened samples to deep cryogenic processing by cooling these from room temperature to 77 K, (b) holding these at 77 K for different durations, e.g., 36 and 84 h, and (c) finally heating the samples back to room temperature. Heating and cooling during deep cryogenic processing were controlled at a uniform rate of 0.75 K/min. Figure 1(b) shows a typical deep cryogenic processing cycle. Temperature control at all stages of heat treatment in QT and QCT was ±2 K and the time gaps between hardening and tempering or between hardening and deep cryogenic processing as well as between deep cryogenic processing and tempering [Fig. 1(a)], were less than 15 min. In order to distinguish the specimens subjected to varied heat-treatment schedules, these are designated here with convenient codes (as shown in Table 1), which have been used in all subsequent discussions. Specimens (≈10× 10 × 7 mm) were cut from the heattreated steel blocks by using wire-cut electro-discharge machining (EDM) for microstructural characterization and hardness evaluation, after careful removal of the decarburized surface layers. These were polished on SiC water-proof abrasive papers of different grit sizes followed by polishing using diamond paste of 3 µm and 1 µm size with the help of an automatic grinding/polishing machine (Autopol-II, METATECH, India). The polished surfaces were etched with Picral solution (3 gm of picric acid in 100 ml ethanol) for revealing the microstructures. The specimens were examined using both optical (Carl Zeiss: Axiovert 40 MAT) and scanning electron microscope (SEM, Jeol: JSM-5510) in secondary electron mode. In addition, energy dispersive X-ray (EDX) micro-analyses were carried out on varied types of the carbide precipitates. Phase analyses were done by X-ray diffraction (XRD) method (PHILIPS: PW 1830 diffractometer with Mo-K radiation) and the volume fraction of the retained austenite was estimated by the suggested procedure in ASTM standard E975-00 [15]. The hardness of the varyingly heat-treated specimens was measured using Vickers indentation test at 30 kgf load. Sliding wear tests were performed according to ASTM standard G99-05 [16] using a pin-on-disc wear testing machine (Make: DUCOM, India, Model: TR 20). Cylindrical specimens of 4 mm diameter and 30 mm length were used as static pins; the rotating counter face was made of tungsten carbide coated En-35 steel disc with roughness value of Ra
