Thermal diffusivity and thermal expansion investigations of WLV steel

WLV steel (32CrMoV12-28) with a density 7.80 g/cm3 at room temperature (RT) was used to study the temperature characteristics of thermal diffusivity and thermal expansion in the temperature range from RT to 1100°C Two types of NETZSCH flash devices were used, i.e. low-temperature LFA 467 and high-temperature LFA 427. A sample of NETZSCH Inconel 600 with a density of 8.35 g/cm3 at room temperature in the temperature range from RT to 480°C was used as the reference material. The reference material was used to determine the thermal conductivity and specific heat by the comparative method. The results of thermal diffusivity tests of WLV steel made with the LFA 467 and LFA 427 apparatus turned out to be consistent with each other. Measurements of thermal expansion of WLV steel were carried out using a NETZSCH DIL 402 C dilatometer. The results of thermal expansion studies indicated a ferrite-austenite phase transition at 835°C, which causes shrinkage of the material. WLV steel has been tested for military use as barrel steel. The paper presents the thermophysical properties of this steel in a wide range of temperatures so that they can be used as input data for numerical simulations of heat transfer in cannon barrels.


Introduction
During a series of shots, the inner surface of the barrel is exposed to the dynamic impact of high temperature and pressure of corrosive propellant gases [1].The inner surface of the barrel is covered with a layer of hard galvanic chrome, protecting against the action of propellant gases [2][3][4].During the operation of the barrel, the chromium layer wears out as a result of high mechanical stress and temperature [1,5].In particular, temperature has a strong effect on the steel of the barrel [1,6,7].The steel layer just below the chromium surface reaches temperatures above the phase transition temperature of α-ferrite into γ-austenite [4,6].For example, for a typical 30HN2MFA barrel steel, the phase transition temperature αγ is about 750℃ [4,6].Austenite grains are characterized by a facecentered cubic (FCC) crystal structure with a denser atom packing in the unit cell compared to bodycentered cubic ferrite grains (BCC) [6,8].Therefore, during the phase transition, shrinkage of the steel occurs and cracks form under the surface of the chromium layer.Due to the cyclical heating and cooling of the steel layer, the cyclical shrinkage of the steel may cause the formation of new cracks in the chromium layer or the expansion of the existing ones [1,7,9].There are two ways to limit this phenomenon.The first is the use of a different steel grade in which there is no ferrite-austenite phase transition.There are few such steels that meet the requirements of the barrel, and they are expensive [10,11].The second solution is the use of steel in which the αγ phase transition temperature is higher than in typical barrel steels [6,10].Steels that exhibit the above feature and at the same time meet the requirements for the barrel are hot work tool steels [8,12].Among these steels is WLV steel (32CrMoV12-28, EN 1.2365).This steel is used for the production of extrusion tools, dies and casting moulds.WLV steel is characterized by high strength at elevated temperatures, high impact strength and resistance to cracking [8,12].Its mechanical properties can be found in the literature, but there are almost no data on thermophysical properties, especially as a function of temperature [12].This paper is an attempt to supplement this information, especially in the case of numerical simulations of heat transfer in the barrel, such as in the papers [7,9,13,14].
The aim of the paper was to determine the temperature characteristic of the thermal diffusivity of the tested steel in the temperature range from RT to 1100℃.The research was also extended to include thermal expansion tests.The obtained results can be used as input data in the heat transfer models in the gun barrels.The temperature characteristics of thermal diffusivity and thermal expansion obtained for the tested steel are original and have not been published before.

Materials
The subject of the research was WLV steel (32CrMoV12-28, EN 1.2365) with a density of 7.80 g/cm 3 at room temperature (RT).The chemical composition of WLV steel is given in table 1.Samples for thermal diffusivity measurements were cut from the supplied material in the form of a disk with a diameter of 12.7 mm and a thickness of 1.63 mm from the material in delivery condition.In order to perform thermal diffusivity measurements, these samples were covered with a layer of graphite in accordance with the recommendations of NETZSCH.The samples for thermal expansion measurements were cut from the same rod as the LFA samples and had the shape of a cylinder with a length of 25 mm and a diameter of 6 mm.

LFA 467 Experiment
Thermal diffusivity measurements were performed using the NETZSCH device.The LFA 467 light flash apparatus enables simultaneous examination of sample and reference material.The reference material was Inconel IN600 and the test material was WLV steel.The density of reference material at room temperature was 8.345 g•cm -3 .Thermal diffusivity was calculated using the standard Cape-Lehman model with pulse correction [15].For this model, the theoretical curve was fitted to the measurement points obtained using an IR (CdHgTe) detector cooling with liquid nitrogen [16].During the measurement, the samples were in an atmosphere of argon as an inert gas.The inert gas flow rate was 20 mL/min.Figure 1a shows the results of thermal diffusivity, thermal conductivity, and specific heat.Tests using reference material allow determining the specific heat and thermal conductivity of WLV steel.The specific heat capacity of the tested material was determined by equation (1) [16]: Equation ( 1) takes into account measurement parameters such as: sample diameter (d) and measurement area of the IR detector (d with Orifice subscript), energy of impulse (Q), amplitude gain of the detector signal (V), density of test and standard material (ρ) and specific heat (cp).The superscript s is associated with sample parameters, and the superscript ref is associated with reference material parameters.The designation T∞ denotes the corrected detector signal.This parameter is proportional to the adiabatic temperature and takes into account heat losses.
Given    () calculated from the equation ( 1) and the thermal diffusivity a(T) obtained from LFA 467, the thermal conductivity k s (T) of the test sample was calculated from the equation (2): where ε() is the relative length change of the sample (thermal expansion).

LFA 427 Experiment
High-temperature thermal diffusivity measurements were performed from room temperature (RT) to 1100℃.High-temperature measurements were carried out using the NETZSCH device: LFA 427 (laser flash apparatus).LFA427 is characterized by the use of a neodymium laser to generate a heat impulse.The parameters of the laser beam are wavelength 1054 nm, pulse energy up to 25 J and pulse width ranging from 100 to 1500 μs.During the experiment, only one sample was placed in the measurement chamber under an inert gas atmosphere (argon).The flow rate was 50 mL/min, and the chamber was vacuumed twice before measurement.Figure 1b shows the thermal diffusivity results in the high temperature range.

DIL 402C Experiment
A NETZSCH pushrod dilatometer (DIL402C) was used to measure the thermal expansion of the tested material.The measurement was carried out from room temperature to 1100℃.In order to take into account the thermal expansion of the dilatometer measuring elements, a calibration measurement was performed using a reference material (NETZSCH sapphire sample -ϕ6 mm, length 25 mm).During the measurements, the same temperature program was used for the tested sample and the reference material (standby at 25℃ -heating to 1100℃ at rate of 2 K/min -isothermal at 1100℃ for 15mincooling down to 25℃ at rate of 2 K/min -isothermal at 25℃ for 15 min).The pressing force of the pushrod on the sample was set to 15 cN.This setting allowed detection of changes in sample length.
The measurement chamber was filled with inert gas and the nitrogen flow rate was 60 mL/min.The thermal expansion of the sample expressed as the coefficient of linear thermal expansion (CLTE) is in practice given in relation to the initial length of the sample L(T0) -CLTE* which is also called by NETZSCH physical alpha (α*) given by the equation (3) [16]: Here, the increase in the length of the tested sample dL(T) takes into account the correction resulting from the change in the length of the pushrod and the sample holder.

Results and Discussion
LFA 467 measurements were performed in the range from RT to 500℃.This experiment showed that no effect related to the release or absorption of heat was observed in the tested material.Figure 1a) shows that the thermal diffusivity and thermal conductivity of WVL steel decrease with increasing temperature.During this time, the value of specific heat increases with temperature.The situation is completely different above 500℃, where the value of the thermal diffusivity coefficient of the tested steel decreases and reaches a minimum at 764.8℃ -figure 1b).Above the Curie point, the thermal diffusivity increases rapidly.Between the temperature of 804.4℃÷822.8℃,we observe a slowdown in the increase in thermal diffusivity.Then, in the range of 822.8℃÷842.8℃,thermal diffusivity increases again, which is associated with the effect of material shrinkage -figure 1b).A summary of the measurement results of LFA 467 and LFA 427 in the temperature range from RT to 480℃ is shown in Figure 1b).The difference between the results is less than 10%.The Nd laser in the LFA 427 generates a uniform pulse compared to the xenon lamp in the LFA 467 and this difference may cause discrepancies in results.For temperatures of 762.7℃ during heating and 757.9℃ for cooling, we observe a small peak corresponding to the lowest value of thermal diffusivity at 764.8℃.The difference between thermal expansion on heating and cooling is significant.During the heating of the sample, we observe the shrinkage of the material for the temperature of 835.8℃ -figure 2b.This peak corresponds to an increase in thermal diffusivity between 822.8℃÷842.8℃and is associated with the ferrite-austenite transition.While cooling, we observe a strong peak at 434.6℃. Figure 3a shows graphs of cooling (temperature), thermal expansion and CLTE versus time.Figure 3b shows a literature plot of the temperature-transformation-time (TTT) relationship and the sample temperature change over time at a cooling rate of 2 K/min.At such a low cooling rate (figure 3b), pearlite and bainite precipitate in the sample [8,17].With reference to figure 3b, in figure 3a the formation temperatures of pearlite (729.6℃) and bainite (456.2℃) are marked.No traces of pearlite precipitates were found on the CLTE diagram for the temperatures shown in figure 3b.The ONSET temperature of 455.1℃ (figure 3a) corresponds to the beginning of the bainitic transformation (456.2℃)-figure 3b). Figure 4 shows a comparison of the results of thermal expansion measurements of WLV steel with a typical 30HN2MFA barrel steel [6].It can be observed that shrinkage for WLV steel occurs at a temperature about 85℃ higher than for 30HN2MFA steel.

Conclusion
The study of temperature characteristics of thermal diffusivity and thermal expansion of WLV steel presented in the paper can be used as input data for numerical simulations of heat transfer in the barrel.The shrinkage effect in WVL steel takes place at a higher temperature compared to the typical barrel steel, which is 30HN2MFA steel [6].In addition, thermal diffusivity and thermal conductivity as a function of temperature are similar to 30HN2MFA steel and relatively high [4,6].This is an advantage because the barrel material with high thermal conductivity better dissipates heat and reduces thermal stress during use of the weapon.The barrel cooling process is more dynamic, therefore the occurrence of phenomena as in the case of cooling at a rate of 2 K/min is unlikely.WLV steel is manufactured for use in the high temperatures and stress conditions that occur in the barrel.It is not stainless steel as the chromium content is too low to dispense with a chromium layer on the inner surface of the barrel, but other surface protection techniques such as nitriding can be tried instead.

Figure 1 .
Figure 1.a) Thermal diffusivity, thermal conductivity, and specific heat of the tested WLV steel; b) thermal diffusivity of WLV steel in a wide temperature range.