Heat Treatment inside the HIP Unit

Abstract

The possibility of combining densification or compaction of steel parts with a heat treatment has recently evolved due to the production of HIP units with a rapid quenching device. Several studies have already been performed to assess the cooling speed and show possibilities for heat-treating steels. It has already been shown that several alloyed steel grades could be hardened by quenching inside a HIP unit. This study aims to characterize the impact of high isostatic pressure during austenitization and quenching on the transformation behavior and resulting microstructure of hardenable steels. The effect of pressure during quenching was studied using two methods. The first method is to measure the latent heat inside the transforming steel during isothermal holding. The release or uptake of energy reveals information about the transformation sequence taking place. The second method is to use the electrical resistivity of a steel as a sensitive indicator for the existing phases and solution state of the steel during continuous cooling after austenitization. Both experimental methods reveal that an isostatic pressure of 170 MPa is sufficient to shift the transformations to longer times and lower temperatures and hence increase the hardenability of hardenable martensitic steel. Introduction A HIP unit was recently introduced that offers the opportunity to quench inside the pressure vessel [1]. Since the introduction of this URQ method (Uniform Rapid Quenching), various research teams have investigated the possibility to heat-treat steel inside a HIP unit. Mashl showed that hardening of a low-alloyed steel inside a pressure vessel leads to higher hardness compared to quenching in oil [2]. The same result was found by Weddeling [3,4]. Angré et al. have shown that a pressure of 170 MPa prolongs pearlite formation in steel specimens that are austenitized and subsequently hold isothermally in the pearlite region [5]. These findings show that a high isostatic pressure of 170 MPa does have an influence on the phase transformations of steel. Therefore, the TTT (Time-Temperature-Transformation) diagrams of every steel are not applicable for HIP heat treatment. In order to correctly predict the hardness and microstructure resulting from an integrated heat treatment in the HIP unit, the TTT diagram as well as the pressure effect must be known. At ambient pressure, the measurement of variations in length during phase transitions (dilatometric measurements) is utilized to determine the temperature and time of a phase transition; however, this was not possible inside a HIP+URQ unit. In the present study, two methods that are capable of indicating phase transitions in theory are tested in a HIP unit and evaluated for two steels. One method, which also was utilized by Angré et al., is to measure the latent heat. The emission or absorption of heat is an indication for a phase transition. The second method of determining a phase transition is to measure the electrical conductivity. It is known from the literature that changes in the electrical conductivity during Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 73-78 doi: http://dx.doi.org/10.21741/9781644900031-10 74 heating or cooling can be related to phase transformations or carbide precipitation [6,7]. Thus, it is of interest to evaluate the method of measuring the electrical conductivity as an indicator for a phase transition during quenching of steels inside a HIP unit. Experimental Materials The latent heat was measured using a cylinder made of AISI H13 (DIN X40CrMoV5-1), and the electrical conductivity was measured using AISI W210 (DIN 100V1). The chemical composition in mass-% of the materials is given in Table 1. Table 1: Chemical composition of the investigated steels. [mass-%] C Si Mn Cr Mo V AISI H13 0.37 0.93 0.32 4.88 1.28 0.92 AISI W210 1.0 0.22 0.1 Heat treatment in the HIP unit The heat treatment was performed inside a hot isostatic press QIH9 with URQ from Quintus Technologies AB. Technical details are given in [4]. The highest pressure at which quenching is possible is 170 MPa. The effect of pressure was analyzed at 25 and 170 MPa; 25 MPa was chosen as a comparatively low pressure that still offers a reasonable quenching efficiency. The heating rate was chosen to be 40 K/min, and heating and pressurizing took place simultaneously. Cooling rates could be changed in three steps (fast, medium, slow) by reducing the volumetric flow rate of the gas by changing the gas inlet nozzle. Measurement of the latent heat during isothermal holding The latent heat was measured using a cylinder of H13 (Ø 70 x 20 mm) with a drilled hole for a core thermocouple (see Fig.1). Fig. 1: Geometry of the H13 cylinder for measuring the latent heat. The black circle shows the position of the core thermocouple. Two further thermocouples were used to control the furnace temperature. The material was austenitized at 1050 °C, held for 30 minutes, and quenched at maximum speed to the isothermal holding temperature. The holding temperatures ranged from 710 °C to 790 °C in increments of Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 73-78 doi: http://dx.doi.org/10.21741/9781644900031-10 75 10 K. According to an isothermal TTT diagram of H13 at ambient pressure, the pearlite transformation takes place in this temperature region. All trials were run directly one after the other without changing the specimen or moving the thermocouples. Measurement of the electrical conductivity Another method of determining an in-situ phase transformation is to measure the electrical conductivity as a function of the cooling temperature. A phase transformation leads to a significant change in the measured electrical conductivity. The temperature and corresponding time at which the transformation takes place are shown by the intersection point of two tangents (Fig. 2). Fig. 2: Determination of the begin and end of phase transformation of W210 in the HIP unit due to changes in the electrical conductivity. Steel W210 was used for this investigation because of its comparatively early pearlite transformation. Preliminary tests showed that the materials used for this measurement must have a significant length to increase the signal intensity. Therefore, wound W210 wire was used. The wire was 300 mm in length and 1 mm in diameter. The feedthroughs for thermocouples were reused as feedthroughs for measuring the electrical conductivity. To exclude the contact and conductor resistances, we opted for the four-wire technique. The electrical conductivity was measured and recorded using the nanovolt-/micro-ohmmeter Keysight 34420 A in combination with BenchVue Digital Multimeter Pro software. The contact points for introducing the current were the ends of the specimen wire, the contact points for measuring the voltage were 10 mm apart from the ends. Contacts were made with spot welding. Further technical details are given in [4]. Quenching trials inside the HIP were performed with two different pressures (25 MPa, 170 MPa) and three different cooling rates. The begin and end of this phase transformation, measured by changes in the electrical conductivity, were compared to a continuous TTT diagram of W210 at atmospheric pressure. Results and Discussion Pressure-induced delay of isothermal pearlite transformation of H13 measured via the latent heat Fig. 3a shows the core temperature of the H13 cylinder during quenching and isothermal holding at 760 °C under two different pressures (25 MPa, 170 MPa). It can be seen that the pearlite transformation takes place due to a significant increase in temperature during holding at 760 °C, Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 73-78 doi: http://dx.doi.org/10.21741/9781644900031-10 76 which results from the latent heat. Furthermore, it can be seen that the maximum of the temperature peak decreases with increasing pressure and is shifted to longer times. The peak temperature correlates to the heat released during the phase transformation of austenite to pearlite. It can be concluded that the amount of released heat correlates to the amount of formed pearlite. This result shows the slowing down and delay of pearlite transformation under pressure. Whereas the begin of the phase transformation can not be measured precisely due to strong undercooling at high pressures, the end of the transformation can be precisely measured as a function of the holding temperature. In Fig. 3b, the end time of transformation is plotted against the holding temperature, which ranges from 700 °C to 770 °C. It can be seen that the end of phase transformation is clearly shifted to longer times. Fig. 3: a) Measured core temperature of the test cylinder of H13 as a function of pressure at a constant furnace temperature of 760 °C. The red marks indicate the end of phase transformation. b) Shifted phase transformation under pressure at different holding temperatures. This effect can be explained by the influence of pressure on the thermodynamic equilibrium according to further investigations at high pressure [8,9]. Therefore, a high pressure stabilizes the phase with a lower molar volume and higher density, respectively. In this case, austenite has a lower molar volume compared to martensite. This leads to an expansion of the austenite phase field and shifts the transformation line to lower temperatures. Therefore, stronger undercooling is needed to initiate pearlite transformation, while at the same time, the diffusion of elements during pearlite transformation slows down due to the lower temperature. The results can be verified with the corresponding microstructure. Fig. 4 shows the resulting microstructure after quenching and holding at 770 °C for 1 hour for 25 MPa (A) and 170 MPa (B). Sample B shows significantly smaller amount of pearlite compared to