Preservation conditions of hot work hardening in die steel with regulated austenitic transformation during exploitation

Introduction

The heat resistance of α-iron-based steels at temperatures around 690 – 700 °C is considered ultimate. Consequently, the most heat-resistant die steels, such as 5Kh3V3MFS, 3Kh2V8F (also known as DIN: X30WCrV9-3, AISI/SAE: H21 or H21A), 4Kh2V5MF, and 4Kh2V4FS, which boast high tungsten content, are typically limited to operating temperatures during hot pressing up to 660 – 680 °C [1 – 3]. Tungsten-free steels like 70Kh3G2FTR or 4Kh5MGFS have even lower operating temperatures [4; 5]. While the operating temperatures of austenitic steels are somewhat higher, their manufacturability is notably low [6 – 8].

In the 1980s, A.D. Ozerskii and A.A. Kruglyakov pioneered the development of die steels featuring a controlled austenitic transformation during exploitation, (RATE steels). These were tungsten-free steels primarily composed of α-iron, designed for high-pressure hot working at operating temperatures reaching up to 750 – 800 °C [9 – 11]. The exceptional durability of these press tools and their extended service life stem from the steels’ capability to maintain hot strain hardening, also known as hot work hardening [12; 13]. This quality distinguishes RATE steels from conventional alloy steels, which are susceptible to softening under high-temperature conditions. The primary cause of this softening lies in the onset of recovery processes and dynamic recrystallization [14 – 16]. As a consequence, there is a notable alteration in the shape of stress-strain curves at elevated temperatures [17; 18].

The inclination towards hot work hardening in RATE steels underwent experimental scrutiny through thermomechanical treatment, involving initial deformation at a lower temperature followed by subsequent deformation at a higher temperature [19 – 21]. The hardening level attained during the preliminary deformation stage was not only sustained but further augmented during the main deformation phase. However, these studies confined the preliminary deformation temperature to 450 °C and the main deformation temperature to 750 °С. Consequently, the temperature ranges conducive to showcasing hot hardening in such steels have not been comprehensively explored. This is a crucial aspect in determining pre-hardening temperatures for the die and operational temperatures that ensure optimal and prolonged die tool performance.

This study aims to investigate the impact of hot deformation temperature on the manifestation of hot hardening in RATE die steel, focusing on a medium-carbon Fe – C – Si – Cr – Ni – Mn –  Mo – V – Ti – Nb steel as an illustrative example.

Materials and methods

In this study, RATE die steel, specifically of the 4Kh2N3M2G4FTBS type [22], was utilized subsequent to a softening heat treatment, resulting in an approximate hardness of ~34 HRC.

Thermomechanical treatment (TMT) was conducted using cylindrical samples measuring 10 mm in height and 5 mm in diameter on a DIL 805 A/D hardening-deformation dilatometer. The TMT process comprised the following sequential stages:

– austenitization at 1150 °C for 15 min;

– a 15 min holding period and preliminary plastic deformation at a temperature range of 400 – 500 °C (in intervals of 50 °C);

– a 15 min holding period and main plastic deformation at a temperature range of 550 – 850 °C (in intervals of 50 °C).

After TMT, the samples underwent free cooling (~10 °C/s).

The layout of the TMT protocol is represented in Fig. 1.

The deformation process was carried out according to a compression sequence involving five cycles, with each cycle involving deformation within the range of 1 – 2 % and a deformation rate set at 1 – 2 %, rate: 0.1 s\(^–\)¹). Process curves capturing “true stress – true deformation” coordinates were recorded throughout the deformation sequences.

The strain hardening index n was calculated utilizing the equation S = Ke\(^{n}\), where S is the true stress; K is the coefficient, and e is the true deformation.

Microstructural analysis of polished sections involved etching in a 5 % aqueous solution of nitric acid. The resulting microstructure was examined using an NIM-100 optical microscope at a magnification of 200x. The grain size was determined from the microstructure images obtained by employing the secant method.

Microhardness was assessed using the Vickers method with a Micromet 5101 Buehler instrument. The experimental parameters were as follows: a load of 300 g, load application time of 10 s, and microscope magnification set at 500x. Measurements were conducted on transverse polished sections of samples subsequent to TMT in two distinct zones: at the periphery and at the center of the sample.

Results and discussion

The mechanical characteristics of the RATE steel during TMT with varying temperatures for preliminary deformation and a consistent temperature for the main deformation are detailed in Table 1, while the strain curves are visually represented in Fig. 2. Similar to earlier investigations [19; 20], multiple plastic deformations at 450 °C led to a notable strengthening of the steel: the maximum cycle stress (S_max) escalated from the initial range of 248 – 263 to 441 – 467 MPa (1.8 times). This achieved level of hardening remained steady during the first cycle of main deformation at 750 °C and further increased across the subsequent four cycles: S_max rose to 517 – 523 MPa (1.1 times). Altering the temperature within the preliminary deformation stage from 400 to 500 °C exerted a marginal influence on the hardening level, both during preliminary and main deformations. At equivalent degrees of deformation, the maximum stress disparity was no more than 6 %. However, this difference diminished as the degree of primary deformation increased. The heightened hardening observed in the first cycle of main deformation, compared to the fifth cycle of preliminary deformation, was most pronounced (10 %) when the preliminary deformation temperature was set at 500 °C.

The mechanical characteristics of the RATE steel during TMT at a constant temperature for preliminary deformation while varying the temperature of the primary deformation are summarized in Table 2, with corresponding strain curves presented in Fig. 3.

The achieved level of hardening during the preliminary deformation stage at a temperature of 450 °C demonstrates intensification during the main deformation phase at temperatures ranging from 550 to 750 °C. Specifically, at 550 °C, S_max increases to 569 MPa (a 27 % increase), while at 750 °C, it reaches 518 MPa (a 15 % rise). Notably, as the temperature of the main deformation escalates from 550 to 750 °C, the propensity for hot hardening diminishes, indicated by a decrease in the strain hardening index ‘n’ from 0.16 to 0.06. Further elevating the temperature of the main deformation to 800 °C does not yield an additional increase in hot hardening; instead, it stabilizes at S_max levels of around 450 MPa (n = 0.01). Eventually, with a subsequent increase in the temperature of the main deformation to 850 °C, some softening of the steel becomes apparent: S_max in the initial deformation cycle drops to 368 MPa (a 20 % decrease), maintaining this level across the subsequent four deformation cycles (n = 0.02). It’s crucial to highlight that even at 850 °C, the S_max values surpass those observed during the initial hardening cycles at 450 °C. Remarkably, the strength level of the RATE steel at 850 °C exceeds that of high-alloy 10Cr–10Ni–5Mo–2Cu steel (under comparable degrees of deformation and loading rates) [23].

The microhardness of the RATE steel after TMT and cooling to room temperature mainly correlates with the level of hot hardening after the main deformation (Fig. 4). Consequently, following preliminary deformation within the range of 400 – 500 °C and subsequent cooling, the microhardness remains constant at approximately 700 HV. After cooling from main deformation temperatures spanning 550 – 800 °C, a minor decreasing trend in microhardness is observed, ranging from 770 to 700 HV. After main deformation at a temperature of 850 °C, the microhardness sharply drops to 580 HV. The disparity in microhardness between the sample’s center and its periphery is negligible.

Fig. 5 illustrates the microstructure, specifically the former austenite grain of the RATE steel after TMT, varying the temperature of preliminary deformation, and cooling to room temperature, alongside histograms depicting grain size distribution.

Table 3 provides the former austenite grain size after TMT, showcasing that an increase in the preliminary deformation temperature from 400 to 500 °C doesn’t influence the former austenite grain’s size, which averages around 28 μm, aligning with 7 points according to State Standard GOST 5639–82.

Fig. 6 illustrates the microstructure, particularly the former austenite grain of the RATE steel after TMT, while varying the temperature of the main deformation and subsequent cooling to room temperature. Additionally, histograms representing grain size distribution are provided.

Table 4 presents the former austenite grain size following TMT at various main deformation temperatures. The data reveals a slight inclination towards an increase in the former austenite grain size from 29 to 35 μm as the main deformation temperature escalates from 550 to 850 °C. This progression aligns with 7 points according to State Standard GOST 5639–82.

Conclusions

The rise in preliminary deformation temperature from 400 to 500 °C minimally impacts the strengthening of steel with RATE at both the preliminary and main deformation stages at a constant temperature of 750 °C.

At a constant preliminary deformation temperature of 450 °C, the level of hardening achieved intensifies during the main deformation stage within the range of 550 to 750 °C. However, this strain hardening tendency weakens with rising temperatures. Further elevation of the main deformation temperature to 800 °C results in a stabilized strengthened state. Subsequently, a marginal softening is observed up to 850 °C.

Increasing the preliminary deformation temperature from 400 to 500 °C at a constant main deformation temperature of 750 °C does not significantly alter the size of the former austenite grain, which averages around 28 μm. Conversely, a subtle increase in the former austenite grain size from 29 to 35 μm is noted when the main deformation temperature rises from 550 to 850 °C, while maintaining a constant preliminary deformation temperature of 450 °C.

The findings suggest that RATE steel demonstrates efficient performance across a broad range of tool heating temperatures, spanning from 550 to 800 °C. Notably, even at a heating temperature of 850 °C, the steel retains a considerably high strength margin of 380 MPa.