Experimental Investigation on Microstructure and Mechanical Properties of Deep Cryogenically Treated Vanadium Alloy Steels

Abstract

In this study, deep cryogenic treatment (DCT) was applied to cold work tool steels with different vanadium weights (Vanadis 4 and Vanadis 10) for 12, 24 and 36 h, and the changes in their mechanical properties and microstructures were examined. Compression, tensile, hardness, SEM–EDS, carbide size, XRD and Rietveld analyses were performed to examine the mechanical and microstructural properties of the cryogenically treated samples. In this study, increasing the cryogenic treatment time and vanadium weight ratio did not have a positive effect on the hardness, and it was determined that the most positive result in terms of tensile and compressive strength was obtained in the V4_DCT-24 sample. The results of this study showed that the cryogenic treatment formed secondary carbides, vanadium carbide (VC) and chromium carbide (Cr₇C₃), in vanadium cold work tool steels and reduced the amount of retained austenite (γ-Fe), transformed into martensite (α’-Fe) structures. Additionally, cryogenically treated Vanadis steels are thought to be usable in the metal processing industry, especially for cutting tools and molds.

1. Introduction

Cold work tool steels are widely used in industrial applications where high wear resistance, strength and toughness are required. These steels typically contain alloying elements such as chromium, vanadium and molybdenum, which increase their mechanical performance and microstructural stability [1]. The primary advantage of cold work tool steels lies in their ability to retain hardness and resist deformation under mechanical stress; this makes them suitable for applications such as forming, stamping and cutting processes [2]. Research has shown that the mechanical properties of cold work tool steels can be significantly improved by various heat treatment (HT) processes, including quenching and tempering, which affect the carbide distribution and preserve the austenite transformation [3]. Additionally, the application of advanced heat treatment processes, including deep cryogenic treatment (DCT), has further improved the mechanical properties of these steels by improving their microstructure and increasing the precipitation of fine carbides [4]. Vanadium element is used together with chromium in high-alloy cold work tool steels and tungsten in high-speed steels. Vanadium is included in the microstructure as a carbide-forming element and is generally used in low amounts (0.025 to 3.2%). Even a very small amount (0.1% by weight) of vanadium alloy added to the structure of steel greatly affects the properties of steel. In general, as the vanadium ratio in the steel composition increases, the amount of carbide formed in the structure also increases, since vanadium is a strong carbide-forming element [3]. They are mostly preferred in cutting tools due to their stable carbide making properties. Vanadium is an element that increases strength, prevents grain growth and extends the life of cutting edges.

Cryogenic treatment (CT), also known as sub-zero treatment, is a special heat treatment process that involves cooling metal parts by exposing them to temperatures below −196 °C using liquid nitrogen [5,6]. Cryogenic treatment is classified as shallow cryogenic (SCT, −50 °C to −80 °C) and deep cryogenic (DCT, −125 °C to −196 °C) treatment, depending on the application temperature. Cryogenic treatments have applications in a variety of industries, including aerospace, automotive and manufacturing. Cryogenic treatment of tool steels is particularly concerned with improving the performance of sheet metal molds, punches, drill bits, cutting tools, pistons and crankshafts subjected to high compression, impact, abrasive wear forces and fatigue life. Cryogenic treatment provides improvement in hardness and wear resistance through microstructural changes and elimination of retained austenite. In addition, the cryogenic treatment transforms the retained austenite into martensite phase, providing homogeneous carbide distribution in the matrix and creating fine carbides [7,8].

In recent years, various studies have investigated the effects of cryogenic treatment on tool steels and have focused on improvements in properties such as hardness, wear resistance and carbide distribution. For example, deep cryogenic treatment in steels such as H13, S5 and Böhler K340 has increased hardness and wear resistance due to martensite formation and refined carbide structure [1,9,10]. However, only a single cryogenic treatment period was used in these studies, changes in alloy composition were not considered and quantitative phase analysis (e.g., the Rietveld method) was not performed. In some other studies on Vanadis 6, Sleipner, W360 and E38K steels, cryogenic temperatures or times were changed; results such as decrease in the retained austenite ratio, increase in carbide density or toughness were reported. However, these studies mostly focused on a single steel type and did not evaluate mechanical properties other than hardness and wear (e.g., tensile or compressive strength) [2,11,12,13]. Although some studies investigated complex alloy steels such as V/Nb or Hf/TaNbC, the effect of vanadium content was not isolated and the treatment time was not systematically changed [3,14]. In addition, some studies have observed improvements by combining DCT treatment with annealing cycles, but still the treatment time and vanadium concentration were not considered as independent variables [15,16,17]. In a study on V alloy ductile iron, the effect of vanadium on phase stability was revealed, but this research was limited to austempering treatment and did not include cryogenic treatment [18].

Unlike previous studies that often focus on a single steel composition and fixed cryogenic treatment durations, this study comparatively investigates two vanadium alloyed cold work tool steels (Vanadis 4 and Vanadis 10) subjected to deep cryogenic treatment (DCT) for 12, 24, and 36 h. While earlier research has demonstrated improvements in hardness and wear resistance through DCT, the effects of varying vanadium content and treatment time two critical factors influencing carbide formation and mechanical performance have not been systematically explored. This study addresses that gap by combining alloy and time based comparisons with quantitative microstructural characterization (SEM–EDS, XRD–Rietveld) and a broader mechanical evaluation including tensile and compressive strength. The findings aim to guide the optimization of DCT parameters for tool steels used in demanding applications such as cutting tools and molds, where a balance of strength, toughness, and microstructural integrity is essential.

2. Materials and Experimental Methods

2.1. Materials

In this study, new generation and patented and Vanadis 4 Extra Superclean^® (V4) (Sollentuna, Sweden) and Vanadis 10 Superclean^® (V10) series cold work tool steels containing different percentages of vanadium produced by Uddeholm company were preferred. The chemical compositions of V4 and V10 steels are given in

Table 1. Chemical composition of Vanadis 4 and 10 steels (wt.%).

	C	Si	Mn	Cr	Mo	V	Fe
V4	1.4	0.4	0.4	4.7	3.5	3.7	85.9
V10	2.9	0.5	0.5	8.0	1.5	9.8	76.8

as weight percentage (wt.%). The base element of the alloy is Fe (iron). The elemental content was determined using SEM–EDS analysis with an accuracy of ±0.5 wt.%.

As in the literature, the austenitizing temperature of vanadium alloy steels varies between 912 °C and 1394 °C [19]. Hardening of vanadium alloy steels is the transformation of austenite, which is formed by waiting for a certain period of time at the austenitizing temperature, into the martensite phase, which has a body centered tetragonal crystal lattice structure, after rapid cooling [11,14]. According to CCT diagrams, the faster the steels are cooled (at a cooling rate that does not interrupt the carbide precipitation line), the more positively the resulting microstructure will affect the performance of the steel. This is achieved by following the cooling curves number 2 in the CCT diagrams given in Figure 1 for V4 and V10 steels. These diagrams are taken from the official technical data sheets for Uddeholm’s Vanadis^® 4 Extra and Vanadis^® 10 cold work tool steels for the general phase transformation or carbide formation behavior of the steels. The carbide formation time is later in V4 steel. Therefore, V4 is expected to be relatively more homogeneous than V10. Thus, it was aimed to increase the amount of retained austenite as the temperature decreased to M_s (martensite starting) temperature. Transforming almost all of the remaining retained austenite into martensite is not possible without an external cooling process after quenching. In addition, the higher M_f (martensite finish) temperature in V4 steel means that less martensite can remain in the structure. In this case, the austenite is completely transformed into martensite. For this reason, after the cryogenic treatment, the steels are tempered and the soft retained austenite is converted into martensite. In general, the microstructures formed that may occur during the cryogenic treatment are given in Figure 2.

2.2. Experimental Methods

Test samples were created with dimensions of Ø10 mm × 10 mm for hardness, SEM–EDS, XRD and Rietveld analyses, and with dimensions of Ø5 mm × 20 mm for the compression test using the CNC lathe of the Emco brand Concept Turn 105 model. Tensile test samples were prepared according to ISO 6892-1 standard [22]. Test samples produced from V4 and V10 steels are shown in Figure 3. Reference samples are called such as with 0 subscripts, only heat-treated (quenching and tempering) samples with HT subscripts, and deep cryogenically treated samples with DCT subscripts, respectively.

The samples were kept in liquid nitrogen environment at −196 °C for 12, 24 and 36 h. The cryogenic treatment was carried out in a “Cryo Production” machine with an internal volume of 80 lt. The process parameters determined according to the CCT diagrams and sample section thicknesses of V4 and V10 steels are given in

Table 2. Heat treatment parameters of V4 and V10 samples.

Sample	Parameters
0 (V4-0 and V10-0)	-	-	-	-	-
HT (V4-1 and V10-1)	Pre-heating at 850 °C for 30 min	Austenitizing at 1080 °C for 30 min	Air cooling at 6 bar pressure	-	Tempering at 560 °C for 60 min
DCT-12h (V4-2 and V10-2)				12 h
DCT-24h (V4-3 and V10-3)				24 h
DCT-36h (V4-4 and V10-4)				36 h

, and the schematic diagram of the heat treatment procedure is given in Figure 4. Cooling and heating processes were carried out gradually (2 °C/min) to prevent the samples from being exposed to thermal shocks and cracks.

The compression test was carried out on Shimadzu Autograph AGS-X brand compression testing device (Kyoto, Japan), with maximum loading capacity of 100 kN and at constant compression speed of 1 mm/min according to ASTM E9 standard [23]. The tensile test was performed on a Shimadzu Autograph AGS-X brand tensile testing machine with a maximum loading capacity of 300 kN at a constant tensile speed of 5 mm/min. The hardness of the samples was tested using the Future Tech brand FR-X2 model Rockwell hardness tester (Kawasaki, Japan). At least five indentation measurements of each sample under 200 g load for 15 s were averaged. Also, standard deviation values were calculated for compression, tensile and hardness tests with the Equation (1). In the symbols in Equation (1), $s$ is the standard deviation, $n$ is the number of measurements, $x_i$ is each observation value and $\overline{x}$ is the average observation value. ANOVA analysis evaluates the relative influence of each factor, allowing a more precise determination of the optimum combination of process parameters. In this study, ANOVA was conducted using MINITAB 15 software at a significance level of 5% to evaluate the contribution of individual factors. Each independent variable in the model is associated with a p-value in the ANOVA table. If the p-value is less than 0.05, the corresponding factor is considered statistically significant. In addition, the last column of the tables containing the compression, tensile and hardness test results present the percentage contribution (%Pc) of each factor, reflecting its effect on the change in mechanical properties.

$$s=\sqrt{{\displaystyle \frac{1}{n-1}}\sum _{i=1}^n{\left(x_i-\overline{x}\right)}^2}$$

(1)

The microstructures of the samples were analyzed with a Hitachi Regulus 8230 scanning electron microscope (SEM) at ×50.000 magnification, and the elemental distributions were analyzed with an energy dispersive spectrometer (EDS). X-ray analysis (XRD) was performed using Panalytical EMPYREAN diffractometer with Cu Kα radiation source. XRD patterns were examined with the help of MAUD (Materials Analysis Using Diffraction, version 2.91) program by using the Rietveld refinement technique. In order to minimize the difference between the examined and calculated diffraction patterns, the harmonization of the data was performed with the Marquardt least squares method. The selection of the most satisfactory harmonization process was made based on the weighted difference error (R_w) in Equation (2).

$$R_{\omega }={\left[{\displaystyle \frac{{\sum }_{i=0}^N{\left(y_i-y_{ic}\right)}^2}{{\sum }_{i=0}^N{y_i\left({2\theta }_i\right)}^2}}\right]}^{1/2}$$

(2)

In Equation (2), y_i represents the observed intensity value obtained experimentally (from the XRD pattern), and y_ic represents the intensity value in the calculated (theoretical) XRD pattern. θ_i is the corresponding Bragg angle for each data point. In Equation (3), reliability of fit (_GOF: goodness of fit) is found by dividing R_w with the estimated error (R_exp).

$$GOF={\displaystyle \frac{R_{\omega }}{R_{exp}}}$$

(3)

In Equation (4),

$$R_{exp}={\left[{\displaystyle \frac{\left(N-P\right)}{{\sum }_{i=1}^N{y}_i}}\right]}^{1/2}$$

(4)

N is the number of examined points and P is the number of coupling parameters. All parameters were corrected by iterative least squares minimization (IRLS). The fact that the compatibility criterion is less than 2 means that the analysis is quite reliable and this coefficient is expressed as S in the program used. Quantitative analysis with Rietveld analysis was performed with the pseudo-Voigt (pV) analytical function, which was determined to be the most suitable for this study. Additionally, in order to numerically determine the average carbide sizes, ×20.000 magnification microstructure images shown in Figure 5 were used and calculations were made according to the formula in Equation (5), using the intercept method and in accordance with the ASTM E112 standard [24].

In Equation (5), l_i is the line length, N_i is the number of carbides that the line intersects, and R is the magnification ratio.

$$d_{ort}={\displaystyle \frac{l_1+l_2+l_3+l_4}{N_1+N_2+N_3+N_4}}·{\displaystyle \frac{1}{R}}={\displaystyle \frac{\sum l_i}{\sum N_i}}·{\displaystyle \frac{1}{R}}$$

(5)

3. Results and Discussion

3.1. SEM–EDS Analysis

SEM analyses shown in Figure 6 were carried out to examine the changes caused by the cryogenic treatment applied to V4 and V10 steels in the microstructures of the samples. SEM images were examined at 2 µm intervals and ×50.000 magnification. There are only ferrite and carbide structures (XRD and Rietveld analyses confirm) as in the structure of the reference samples (V4-0 and V10-0). The carbides in the samples are coarsely distributed. Since the tempering process was not applied to these samples, no martensite phase was detected in the microstructure.

SEM images of V4 samples show that the carbides are distributed more homogeneously and spherically, and the carbide sizes are smaller. This is also supported by the carbide size measurement results. In V4 samples, as the cryogenic treatment time increased, the martensite structure surrounded the carbides like a network. SEM images of V10 samples show that as the cryogenic treatment time increases, finely dispersed carbides bind together, carbide structures become irregular and sphericality decreases. Joint defects formed in the gaps around the carbides and between the tempered martensite phase. It is thought that microcracks occur due to the restructuring of amorphous and large carbides in the microstructure. V10 samples are larger in terms of grain size than V4. This is associated with the higher tendency for carbide aggregation in the grains of V10 samples. Microdefects formed between the carbide and matrix during rapid cooling create problems in grain aggregation. These microcracks reduce the mechanical properties [11,25,26]. In high-alloy steels, carbides formed by alloys dominate the structure transformations. SEM images of V10 samples showed that finely dispersed carbides bonded together, carbide structures became irregular, and sphericality decreased [8,27,28].

SEM analysis results clearly showed that as the cryogenic treatment time increased, the carbides exhibited finer and more homogeneous distributions. It is also supported by Rietveld analysis data that martensite phases, primary and secondary carbides appear in the structure with the tempering process. It was concluded that the white areas in the microstructure decreased and the carbides in the structure became evident due to the transformation of retained austenite into martensite [29,30,31].

EDS analysis was performed to determine the presence of elements in the SEM micrographs of the samples before and after the cryogenic treatment. The EDS spectra presented in Figure 7 belong to the V4-DCT24 and V10-DCT24 samples, which show the most positive results in terms of tensile and compressive strength, and the analysis determined that the samples generally contained Fe, Cr, V, C and Mo elements. XRD analysis results also support the formation of iron, chromium, vanadium, carbon elements. SEM–EDS analyses of V4 and V10 steels revealed that the matrices are Fe dominated and due to the high alloying element content, the dark spherical areas in the microstructure are V-dominated carbides, and the lighter spherical areas are Cr-dominated carbides. As the cryogenic treatment time increased, general increase in the Cr and V phases was detected.

3.2. Carbide Size Measurement

The average total carbide sizes obtained by the intercept method from the microstructure images of V4 and V10 samples, and the grain sizes of VC and Cr₇C₃ carbides in the microstructure are given in Figure 8.

As shown in the SEM images and Figure 8, the grain sizes of the carbides in the V4-0 reference sample are quite small, while they are larger in the V10-0 reference sample. This situation is associated with the formation of both VC and Cr₇C₃ carbides since the VC ratio in V4 steel and the Cr ratio in V10 steel are high. The average grain sizes of the V4-1 and V10-1 samples are observed to increase with quenching and tempering processes. As the cryogenic treatment time increases, the VC and Cr₇C₃ carbide grain sizes in the V4 samples generally decrease and show a homogeneous distribution. It is understood that the carbides formed in the V10 samples combine over time, their sphericality deteriorates and the combined carbides tend to become larger.

3.3. XRD and Rietveld Analysis

The phases in the structure from the X-ray diffraction (XRD) diagrams were determined to examine the changes in the structure of V4 and V10 steels during the cryogenic treatment. In Figure 9 and Figure 10, the patterns of samples V4 and V10, martensite (α′-Fe), retained austenite (γ-Fe) and peaks of spherical carbide compounds in the form of VC and Cr₇C₃ were determined together with the diffraction planes.

According to the XRD patterns of V4 samples, no austenite structure observed in V4-0, V4-3 (Q + DCT₂₄ + T) and V4-4 (Q + DCT₃₆ + T) samples. Sample V4-0 consists entirely of ferrite phase. In the XRD patterns of V4-1 (Q + T) and V4-2 (Q + DCT₁₂ + T) samples, it was observed that the intensity of austenite peaks decreased, while the intensity of martensite peaks first increased and then decreased slightly.

According to the XRD patterns of V10 samples, no austenite structure observed in the V10-0 sample. Sample V10-0 consists entirely of ferrite phase. Compared to the V4 sample results, the highest amount of retained austenite showed in V10 steels. A large amount of ferrite transformed into Cr₇C₃ carbides, increasing the amount of Cr₇C₃ compared to V4 [27,28,29,30,31,32,33].

The results obtained from Rietveld analysis regarding the amounts and crystallographic properties of the phases present in the structure are given in

Table 3. Rietveld analysis results (a, b, c: lattice parameters).

Sample	Martensite	a	b	c	Austenite	a	VC	a	Cr7C3	a	b	c	Sigma
V4-0	82.89 (*)	2.878	2.878	2.878	-	-	9.73	4.182	7.38	4.552	7.038	12.177	1.184
V4-1	90.95	2.906	2.906	3.392	0.64	3.607	5.46	4.192	2.95	4.530	6.976	12.110	1.132
V4-2	88.14	2.893	2.893	3.138	0.25	3.600	9.39	4.183	2.22	4.534	6.973	12.070	1.185
V4-3	86.39	2.891	2.891	3.046	-	-	11.23	4.182	2.38	4.534	6.975	12.080	1.142
V4-4	87.45	2.894	2.894	3.131	-	-	10.39	4.183	2.16	4.542	7.002	12.005	1.148
V10-0	74.99 (*)	2.879	2.879	2.879	-	-	16.56	4.169	8.43	4.514	7.027	12.264	1.203
V10-1	74.41	2.892	2.892	3.283	1.18	3.585	20.39	4.164	4.02	4.520	7.043	12.217	1.181
V10-2	73.56	2.887	2.887	2.938	0.91	3.589	22.42	4.166	3.11	4.521	7.046	12.200	1.176
V10-3	75.10	2.891	2.891	2.956	0.89	3.589	21.12	4.167	2.89	4.522	7.038	12.203	1.214
V10-4	72.85	2.837	2.837	2.905	0.85	3.585	23.18	4.163	3.12	4.521	7.040	12.200	1.289

(*) Ferrite Sigma: Accuracy degree of the analysis (expected to be between 1 and 2 and close to 1).

.

According to the Rietveld analysis results, it was determined that the amount of retained austenite decreased with the cryogenic treatment. The cryogenic treatment appears to transform the retained austenite in the microstructure into martensite [34,35]. This is in agreement with the literature [5,9,15]. In Table 3, it is observed that as the cryogenic treatment time increases, there are slight decreases in both the retained austenite amount and the % carbide (Cr₇C₃) ratio. The amount of retained austenite and Cr₇C₃ carbides, which are harder than VC carbides, directly affect the hardness [14,36].

3.4. The Compression Test

The compression test was performed to determine the mechanical properties of V4 and V10 steels applied with different cryogenic treatment times. The average values of maximum compressive strength, strain, and elastic modulus obtained from three samples for each processing condition are presented in

Table 4. Compression test results.

Sample	Compressive Strength (MPa)	Standard Deviation (MPa)	Compressive Strain (%)	Modulus of Elasticity (MPa)
V4-0	932.33	19.15	11.18	83.39
V4-1 (Q + T)	2968.80	156.77	7.33	405.02
V4-2 (Q + DCT12 + T)	2637.16	49.25	6.75	390.69
V4-3 (Q + DCT24 + T)	2801.73	58.2	7.43	377.08
V4-4 (Q + DCT36 + T)	2410.33	23.12	5.83	413.44
V10-0	1104.29	28.77	12.45	88.70
V10-1 (Q + T)	2913.46	125.97	8.03	362.82
V10-2 (Q + DCT12 + T)	2657.10	69.6	6.99	380.13
V10-3 (Q + DCT24 + T)	2347.73	131.97	6.31	372.06
V10-4 (Q + DCT36 + T)	2687.48	78.63	7.73	347.67

. Compression test application and images of samples after the test are given in Figure 11.

Stress–strain curves of samples V4 and V10 are given in Figure 12a,b.

During the compression tests, all samples were kept under load until significant plastic deformation was reached. The degree of deformation (engineering strain) values at the moment of maximum compression strength were calculated with Equation (6) in the range of approximately 12% (min) to 18% (max) depending on the sample.

$$\epsilon ={\displaystyle \frac{∆L}{L_0}}={\displaystyle \frac{L-L_0}{L_0}}$$

(6)

In V4 samples, the highest maximum stress showed in the V4-1 (Q + T) sample, and the highest strain showed in the V4-0 sample, which is the reference sample. The most optimum effect of the cryogenic treatment was obtained in the V4-3 (Q + DCT₂₄ + T) sample. In V10 samples, the highest maximum stress showed in the V10-1 (Q + T) sample, and the highest strain showed in the V10-0 sample, which is the reference sample. The most optimum effect of the cryogenic treatment was obtained in the V10-4 (Q + DCT₃₆ + T) sample.

Table 5. ANOVA analysis for compressive strength.

Factor	Dof	F	p Value	Pc (%)
Vanadium wt.% (A)	1	35.84	0.002	38.26
Cryogenic time (B)	2	21.15	0.005	29.77
Tempering (C)	1	14.68	0.008	17.34
A (V wt.%) × B (Time)	2	9.45	0.021	8.13
A (V wt.%) × C (Temp.)	1	4.33	0.067	2.5
B (Time) × C (Temp.)	2	2.19	0.131	1.46
Error	10	-	-	2.54

shows that the p-value of all three factors is less than 0.05, that is highly significant at the 95% confidence interval. It can be observed that vanadium weight (38.26%) is the main contributing factor, followed by cryogenic time (29.77%) and finally tempering (17.34%) and affects the compressive strength of these alloys.

As the compressive strength values in V4 and V10 steels increase, the strain (elongation at break) decrease. Defects in the structure of V10 steel and microcracks caused the hardness and compressive strength values to decrease. Compared to V4 steel and V10 steel, the high amount of chromium element in the chemical composition of V10 steel is thought to negatively affect the mechanical properties.

3.5. The Tensile Test

The Tensile test results were obtained by taking the average values of cryogenically treated V4 and V10 tensile samples. The average values of tensile strength, yield strength, strain and standard deviation obtained from three samples for each processing condition are presented in

Table 6. Tensile test results.

Sample	Ultimate Tensile Strength (MPa)	Standard Deviation (MPa)	Yield Strength (MPa)	Relative Elongation (%)
V4-0	806.23	11.23	697.39	12.84
V4-1 (Q + T)	2040.94	26.83	1928.48	15.04
V4-2 (Q + DCT12 + T)	2042.60	27.36	1860.99	13.91
V4-3 (Q + DCT24 + T)	2125.47	25.64	1914.69	14.11
V4-4 (Q + DCT36 + T)	2110.51	27.89	1921.20	12.41
V10-0	809.39	10.42	947.87	11.07
V10-1 (Q + T)	962.62	19.48	860.00	11.18
V10-2 (Q + DCT12 + T)	1342.00	25.51	1063.00	24.52
V10-3 (Q + DCT24 + T)	1345.00	25.71	1063.48	25.13
V10-4 (Q + DCT36 + T)	1129.00	22.58	892.12	20.22

.

Table 7. ANOVA analysis for tensile strength.

Factor	Dof	F	p Value	Pc (%)
Vanadium wt.% (A)	1	210.14	0.000	36.35
Cryogenic time (B)	4	0.00	1.000	0.00
Tempering (C)	1	219.25	0.000	37.96
A (V wt.%) × B (Time)	4	9.98	0.012	10.33
A (V wt.%) × C (Temp.)	1	53.92	0.002	9.34
Error	8	-	-	6.02

shows that the p-value of all three factors is less than 0.05, that is highly significant at the 95% confidence interval. It can be observed that tempering (37.96%) is the main contributing factor, followed by vanadium weight (36.35%) and finally cryogenic time (0.00%) and affects the tensile strength of these alloys.

In V4 samples, the highest tensile strength was observed in the V4-3 (Q + DCT₂₄ + T) sample, closely followed by V4-4 (Q + DCT₃₆ + T). The highest relative elongation was obtained in the V4-1 (Q + T) sample, suggesting that conventional quenching and tempering improved both strength and ductility. Among the cryogenically treated samples, V4-3 showed a balanced improvement in both strength and elongation, indicating that 24 h of DCT provided the most favorable effect in V4 samples. In V10 samples, the results revealed a different behavior. Although the highest tensile strength values were observed in V10-2 (Q + DCT₁₂ + T) and V10-3 (Q + DCT₂₄ + T), the most remarkable change was in the elongation values, which increased significantly to over 24%, indicating enhanced ductility after cryogenic treatment. Figure 13 shows representative SEM images of fracture surfaces of Vanadis steels. These findings confirm the SEM images in this study.

V10 steel showed more ductility after cryogenic treatment than V4. The improvements in V4 were mostly in strength. A decrease in both strength and elongation values was observed at long treatment times (36 h). This suggests that long-term cryogenic treatment may lead to brittleness due to excessive carbide precipitation or microstructural stress accumulation.

3.6. The Hardness Test

Average hardness and standard deviation values were determined by taking measurements from five different regions of each of the three samples for each processing condition, and the results are presented in

Table 8. Hardness test results.

Sample	Average Hardness (HRC)	Standard Deviation (HRC)
V4-0	39.68	0.2125
V4-1 (Q + T)	66.15	0.2463
V4-2 (Q + DCT12 + T)	65.90	0.2760
V4-3 (Q + DCT24 + T)	66.90	0.1953
V4-4 (Q + DCT36 + T)	66.25	0.3162
V10-0	40.33	0.1800
V10-1 (Q + T)	67.30	0.3482
V10-2 (Q + DCT12 + T)	64.75	0.2928
V10-3 (Q + DCT24 + T)	65.40	0.2451
V10-4 (Q + DCT36 + T)	66.10	0.3217

and Figure 14. The lowest hardness values in V4 and V10 steels showed in V4-0 and V10-0 samples reference samples (without any treatment). In V4 steels, the highest hardness value was obtained in the V4-3 (Q + DCT₂₄ + T) sample, and then in the V4-4 (Q + DCT₃₆ + T) sample. For V4 samples, as the cryogenic treatment time increased, the hardness first increased and then decreased. In V10 steels, the highest hardness value was obtained in the V10-1 (Q + T) sample, and then in the V10-4 (Q + DCT₃₆ + T) sample. For V10 samples, hardness values increased as the cryogenic treatment time increased.

As seen in

Table 9. ANOVA analysis for hardness.

Factor	Dof	F	p Value	Pc (%)
Vanadium wt.% (A)	1	10.008	0.00297	0.01
Cryogenic time (B)	4	20,191.049	0.00000	67.38
Tempering (C)	1	38,871.418	0.00000	32.43
A (V wt.%) × C (Temp.)	4	42.547	0.00000	0.14
Error	40	-	-	0.03

Dof—degree of freedom; Pc—percentage of contributions.

and Figure 14, no statistically significant difference was observed according to standard deviation values and ANOVA results. However, the most optimum hardness values were generally obtained in V4 steels.

Table 9 shows that the p-value of all three factors is less than 0.05, that is highly significant at the 95% confidence interval. It can be observed that cryogenic time (67.38%) is the main contributing factor, followed by tempering (32.43%) and finally vanadium weight (0.01%) and affects the hardness of these alloys.

Hardness test results show that the hardness values of cryogenically treated samples vary due to secondary carbide precipitation of the microstructure. It was confirmed by Rietveld analysis that the decreases and increases in hardness values resulted from the dissolution and precipitation phases of some MC (Mo, V, W, Cr) carbides. In addition, the uncertain increases or decreases in hardness values caused by cryogenic treatment applied to V4 and V10 steels after heat treatment are also supported by the literature [10,11,12,13,14,15,16,17,18,20,21].

4. Conclusions

In this study, the effects of different cryogenic treatment times (12, 24 and 36 h) on the mechanical and microstructural properties of Vanadis 4 Extra Superclean^® (V4) and Vanadis 10 Superclean^® (V10) series cold work tool steels were investigated. Compression, tensile, hardness, SEM–EDS, carbide size, XRD and Rietveld analyses of cryogenically treated samples were performed.

This study does not aim to replace conventional treatment with DCT, but to systematically understand the effect of cryogenic treatment time on mechanical and microstructural properties depending on alloy composition. Unlike the studies in the literature, in this study, V4 and V10 steels, which differ significantly in terms of V and Cr content, were evaluated simultaneously, thus making a composition-based comparison. The study results show that 24 h cryogenic treatment significantly improves the tensile behavior for both steel types. This finding shows that DCT can be used as a selective tool to optimize certain mechanical properties depending on alloy composition, which is especially valuable for industrial applications where strength-toughness balance is required. Compression tests clearly show the effect of microstructural differences on mechanical behavior. Small carbides and regular grain structure obtained after 24 h cryogenic treatment in V4 steel resulted in high compressive strength and increased elastic modulus. On the other hand, large carbides and microcracks observed in V10 steel after 36 h treatment were associated with a decrease in load carrying capacity and deterioration in deformation behavior. The limited change in hardness values is a typical situation observed in high-alloy steels after tempering and can be explained by microstructural changes such as the transformation of remaining austenite into martensite and the precipitation of Cr₇C₃ carbides [14,27,28]. However, the highest hardness values among the samples were generally obtained in V4 steels. For this reason, it has been concluded that V4 steels should be chosen in areas where use is required in terms of hardness. Most importantly, in this study, quantitative microstructure data were presented by XRD–Rietveld and SEM–EDS analyses; thus, the decrease in the amount of remaining austenite (γ-Fe), the formation of martensite (α’-Fe) and the changes in carbide morphology were clearly revealed [7,13,29]. SEM images and EDS results showed that small, spherical and homogeneously distributed VC carbides were formed especially in V4 steel by increasing the cryogenic treatment time up to 24 h; whereas in V10 steel, larger, irregular and angular Cr₇C₃ carbides became dominant due to the increasing Cr content. These carbides grew significantly in 36 h of treatment, and loss of sphericality and carbide agglomeration were observed especially in V10 samples. This situation increases the risk of stress concentration and microcrack formation at grain boundaries. It was also evaluated that these changes in the microstructure also affect the grain size, especially in V4 steel, a finer and more homogeneous grain structure was observed due to the late transformation carbide formation.

In conclusion, this study shows that the effectiveness of the cryogenic treatment depends on both the chemical composition and the treatment time and provides concrete guidance for optimizing the DCT parameters for different steel grades. In this study, the cryogenic treatment was found to be effective but not for very long treatment times (more than 12 h). It was observed that in case of vanadium content more than 4% (in V10 steels), cryogenic treatment time did not affect these steels at least in a positive way. This is associated with the relatively larger size of the carbides formed in V10, their higher bonding tendency and microdefects in the microstructure that cause problems during bonding. The tendency of these microdefects to form microcracks is supported by the SEM images obtained in this study and the literature [13,18,21]. The findings show that cryogenically treated Vanadis steels can be directly used in applications in the metalworking industry, especially in cutting tools and mold manufacturing of that require high hardness and toughness.