Influence of austenitization time on the grain size and mechanical properties of a 5Cr–0.5Mo steel

1 Introduction

Chromium–molybdenum (Cr–Mo) steels are extensively used in systems subjected to high-pressure and high-temperature conditions, including petroleum cracking facilities, coking units, fertilizer production systems, steam plants, petrochemical facilities, and power plants [1], [2], [3]. The widespread use of these steels is attributed to their high resistance to oxidation and creep when exposed to temperatures of 350–600 °C and pressures of 15–30 MPa during continuous operation for at least 250,000 h [4], [5], [6]. Cr–Mo steels are categorized into various grades based on their Cr–Mo content [7] and classified into ferritic and austenitic steels according to their tensile strength (S _u ) [8]. Specifically, 5 % Cr–0.5 % Mo (5Cr–0.5Mo) steel, an intermediate alloy steel [8], is widely used in the petrochemical industry owing to its high S _u value and resistance to corrosion induced by hydrogen sulfide (H₂S) and other agents in crude oil [5]. This steel is also employed in the piping systems of oil refining furnaces [9] and is a common choice for heat exchangers and pressure vessels [10], [11]. The excellent mechanical and chemical properties of 5Cr–0.5Mo steel – such as strong resistance to creep [4], [12], high-temperature corrosion resistance, and resistance to hydrogen attacks in H₂S-containing environments – drive ongoing engineering research aimed at further characterizing this alloy [5], [13].

Extensive research has been conducted on steel alloys to elucidate their microstructural transformations during heat treatments [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], and prolonged exposure to elevated temperatures during long-term service [5], [7], [27], [28], [29]. These studies are crucial because microstructural alterations significantly influence the mechanical properties of steel alloys [2], [6], [17], [22], [30], [31]. In particular, 5Cr–0.5Mo alloy steel must not be supplied in its as-rolled condition, as its untreated metallurgical structure will be martensitic, resulting in very poor toughness and extremely high hardness. To ensure optimal mechanical properties, piping materials made from 5Cr–0.5Mo steel should be provided in one of the following forms: annealed, isothermally annealed, or normalized and tempered [2]. Additional research has explored the phase transformations of 5Cr–0.5Mo alloy steel during heat treatments and upon exposure to operating temperatures [30], [32]. Such studies are essential for understanding the relationship between the microstructure and mechanical properties of this steel under varying operational conditions.

Annealing, normalizing, and hardening are essential heat treatment processes used to alter the mechanical properties of engineering materials. These processes improve hardness, ductility, corrosion resistance, creep resistance, S _u , yield strength (S _y ), and machinability while facilitating the precise adjustment of mechanical properties to meet specific design requirements [17].

Rahman et al. [16] investigated the effects of hardening, normalizing, and full annealing on low-carbon steel. Their findings revealed that the hardening process produces a martensitic structure, resulting in extremely hard steel. In contrast, full annealing creates a ferritic structure with lower hardness, while normalizing provides moderate hardness and ductility. Thomason [15] investigated the effect of heat treatment on the ductility of alloy steel wires. Their results indicated that annealing at 700 °C for 1 h, followed by air cooling, resulted in optimal ductility. They concluded that no general correlation exists between decreasing hardness and increasing ductility in steel wires. Chandra et al. [17] studied the effects of heat treatment on the properties and microstructure of steels. They reported that annealed specimens with predominantly ferritic structures exhibited the lowest S _u and hardness values but the highest ductility and toughness values. In contrast, hardened samples composed of martensite displayed the highest S _u and hardness values but the lowest ductility and toughness. Yu et al. [33] investigated the effect of Jominy distance on the crystallography and hardness of a steel bar quenched from 900 °C. They observed that hardness decreased significantly with increasing Jominy distance and noted a near-linear relationship between high-angle grain boundaries and hardness. Nalcaci et al. [34] examined the effects of short-time austenitization on the mechanical properties and transformation-induced plasticity of austempered ductile iron. They reported that increasing austenitization time led to higher carbon content in the austenite, an increase in bainitic ferrite cell size, and changes in the phase volume fractions of austenite transformation products. These changes resulted in an approximate 12 % improvement in S _u and percentage elongation (δr) after 15 min of austenitization compared to the properties achieved after 1 h. Mudasiru et al. [35] evaluated the impact of immersion speed on the mechanical properties and microstructure of oil-quenched AISI 1020 steel. They found that increasing quenching speed reduced S _y and δr but increased Young’s modulus. Senthilkumar and Ajiboye [36] studied the effects of various heat treatments on medium-carbon steel. They reported that tempered specimens exhibited the highest S _y , followed by hardened, normalized, and annealed specimens. Moreover, hardened specimens demonstrated the highest ultimate strength, followed by tempered, normalized, and annealed specimens.

The above investigations constitute the broader research on steels aimed at understanding their mechanical properties under various heat treatments. The primary objective of this study is to assess how varying austenitization times influence the grain size and static mechanical properties of 5Cr–0.5Mo steel during extended heat treatment. To this end, a nonagitation water quenching heat treatment (WQHT) was performed at 900 °C with austenitization times of 1, 2, 4, 6, and 8 h to evaluate grain size growth and microstructural changes. Based on the observed grain sizes, three austenitization times (1, 4, and 8 h) were selected, and normalizing heat treatment (NHT) was applied, allowing the samples to air-cool without stirring. Finally, the microstructure of the specimens was examined, and mechanical properties were assessed through hardness, microhardness, and tensile tests to determine S _y , S _u , area reduction (%Ar), and %δr at break.

2 Materials and methods

An oilfield steel pipe made from 5Cr–0.5Mo steel, standardized as ASTM A387 Gr5 [37], with an external diameter of 169 mm and a thickness of 12.5 mm [38], [39] was analyzed in this study. Atomic absorption spectroscopy (C and S analysis) and energy-dispersive X-ray spectroscopy [11], [13], [39] were used to verify the chemical composition of the steel in its as-received condition.

WQHTs were applied to five specimens with five replicates per sample with dimensions of 120 mm × 20 mm × 9.55 mm. The specimens were austenitized at 900 °C for five different durations (1, 2, 4, 6, and 8 h) and then cooled in water at room temperature without stirring, using a cylindrical metal container with a radius of approximately 300 mm and a height of 500 mm. Among these specimens, those subjected to austenitization for 1, 4, and 8 h were selected for NHT for identical durations (N1HA: normalized after 1 h of austenitization, N4HA: normalized after 4 h of austenitization, N8HA: normalized after 8 h of austenitization) and subsequently cooled in air without agitation (see, Table 1).

The grain sizes of the specimens in the as-received condition, after WQHT, and after NHT were evaluated by metallographic analysis following the ASTM E3-01 [40] procedure. The preparation steps were as follows: specimens subjected to WQHT were sectioned using a Struers cutter (model Loboton) and mounted in a Buehler Ltd., mounting press (model Pneumet 1). Coarse grinding was performed with 180-grit emery paper on a Leco Belt Grinder (model BG-20), followed by fine grinding with emery papers of 240, 300, 400, and 600 grit using a Leco Grinder-Polisher (model GP-25). Final polishing was carried out with 3 µm and 1 µm diamond pastes on a Struers polisher (model Kmeth-Rotor-3). Etching was conducted with Vilella’s reagent (1 g picric acid, 5 mL HCl, and 100 mL ethanol) in accordance with ASTM E407 [41]. For specimens in the NHT and as-received conditions, the same procedure was applied, except that etching was performed with 5 % Nital (5 % HNO₃, 95 % ethanol). Microstructural characterization was performed using a Union optical microscope (model Versdamet-2) at 100X magnification and a Union stereomicroscope (model 072073). Grain size was determined according to ASTM E930 [42], by measuring the cross-sectional size of the largest grains observed in the metallographically prepared sections, considering the presence of occasional coarse grains.

To assess the mechanical properties of the 5Cr–0.5Mo steel specimens subjected to WQHT, Rockwell C hardness (HRC) and Vickers microhardness (HV) tests were conducted in accordance with ASTM E18-08b [43] and ASTM E384 [44] standards. For the as-received and normalized specimens, Rockwell B hardness (HRB), HV, and tensile tests were performed according to ASTM E10 [45], ASTM E384 [44], and ASTM E8-09 [46], respectively. Tensile tests were performed using an MTS universal testing machine (25-t capacity). Three specimens were loaded under displacement control at a rate of 7.0 mm min⁻¹ until failure, with axial deformation measured using an INSTRON extensometer with a 1-inch gauge length. The applied load and extensometer displacement were recorded during each test using a data acquisition system connected to the universal testing machine. The stress‒strain curves were plotted from the test results, and mechanical properties such as S _y , S _u , %A, and %δr were measured.

Finally, a fracture surface analysis was conducted on tensile specimens in the as-received state and after normalization treatment: Briefly, a 10 mm section was cut from the surface of the specimens, cleaned ultrasonically with alcohol for 20 min to remove impurities, and dried. Images were then captured using scanning electron microscopy (SEM, JEOL model JSM 6390).

3 Results

3.2 Microstructure

Figure 1 presents the microstructure and grain size results for WQHT specimens subjected to austenitization for 1, 2, 4, 6, and 8 h. Notably, the microstructure of the WQHT specimens subjected to different heat treatment times primarily features martensite and martensite with retained austenite. Mohapatra et al. [13] also reported a similar microstructure. Figure 1 illustrates a direct relationship between austenitization duration and grain size; specifically, grain size increases with longer austenitization times. Additionally, the grain size was homogeneous across all samples. According to the ASTM E930 [42] standard, the grain sizes (ALA) of WQHT specimens austenitized for 1, 2, 4, 6, and 8 h (WQ1HT, WQ2HT, WQ4HT, WQ6HT, WQ8HT) were 3.5, 3.0, 2.0, 2.0, and 1.5, respectively. These results confirm the trend of increasing grain size with longer austenitization times. Based on the measured grain sizes, three representative austenitization times (1, 4, and 8 h) were ultimately selected for NHT.

Figure 1:

Microstructure of 5Cr–0.5Mo steel specimens subjected to WQHT for various austenitization times, a) 1 h (WQ1HT), b) 2 h (WQ2HT), c) 4 h (WQ4HT), d) 6 h (WQ6HT), and e) 8 h (WQ8HT) (X100 magnification).

Metallographic analysis of the as-received steel and steel specimens after austenitization for 1, 4, and 8 h (Figure 2) was conducted using optical microscopy to examine the microstructure. Figure 2a presents a ferritic matrix with dispersed carbides, consistent with the observations of Rivas et al. [9] and Mohapatra et al. [11]. Figure 2b–d illustrate the microstructures of the specimens subjected to NHT after 1, 4, and 8 h of austenitization, respectively. In all three cases, the microstructure primarily features a mixture of bainite and martensite. Longer austenitization times result in larger bainite size, which, in turn, increases the overall grain size. The control of bainitic morphology through careful adjustment of austenitization and transformation parameters has also been demonstrated in wear-resistant bainitic overlays for rail steels, where carbide-free bainite improved both toughness and wear resistance [47].

Figure 2:

Microstructure of 5Cr–0.5Mo steel, a) as-received, b) N1HA, c) N4HA, and d) N8HA (X100 magnification).

3.3 Mechanical properties

Table 2 lists the results of hardness measurements for 5Cr–0.5Mo steel in the as-received state, after exposure to WQHT, and following exposure to NHT.

Table 2:

Hardness and microhardness data of 5Cr–0.5Mo steel specimens under different conditions.

Material as-received		Hardness (HRB)	Microhardness (HV)
		71.0 ± 0.7	151.5 ± 1.8
Heat treatment	Austenitization time (h)	Hardness (HRC)	Microhardness (HV)
WQHT	1	34.1 ± 0.5	333.6 ± 10.3
	2	30.7 ± 1.2	325.0 ± 12.1
	4	30.9 ± 0.5	353.0 ± 14.4
	6	29.0 ± 0.8	311.0 ± 8.6
	8	28.9 ± 0.5	300.5 ± 8.7
NHT	1	29.3 ± 1.4	381.2 ± 9.4
	4	33.4 ± 0.5	401.6 ± 14.5
	8	32.6 ± 1.7	317.2 ± 6.6

The hardness variations observed in Table 2 can be interpreted in terms of the microstructural changes promoted by different austenitization times. In the WQHT condition, the high hardness after 1 h of austenitization is associated with a fine martensitic structure and the presence of partially dissolved carbides, which act as strong barriers to dislocation motion. The gradual decrease in hardness for longer holding times (2 h and ≥ 6 h) is explained by grain coarsening and the reduction of grain boundary density, leading to lower dislocation resistance according to the Hall–Petch relationship [4], together with further carbide dissolution that reduces the number of effective obstacles to plastic deformation [5].

Interestingly, the increase in hardness observed at 4 h (353 HV) does not follow the general decreasing trend. This peak can be attributed to a specific microstructural balance: at this intermediate time, grain growth is not yet excessive, but the redistribution of M23C6 and M6C carbides and local carbon enrichment in the austenite enhance the subsequent formation of harder martensite during quenching [11], [30]. Furthermore, a slight reduction in retained austenite fraction compared to the 2 h condition likely contributes to this higher hardness. Similar nonlinear effects of intermediate austenitization times on mechanical properties have also been reported in other ferrous alloys [34]. A similar nonlinear trend in mechanical response has also been reported in dual matrix austempered ductile iron (DMS-ADI), where an intermediate balance between ausferrite and retained austenite fractions led to maximum strength and hardness at specific austempering conditions [48]. This effect can be attributed to carbon redistribution between bainitic ferrite and high-carbon austenite, which modifies the martensite morphology formed upon cooling, in agreement with observations in partitioned austempered ductile irons [49].

After NHT, all specimens display significantly higher hardness compared to the as-received state, owing to the transformation into a bainite–martensite mixture with a high density of dislocations and finely distributed precipitates that hinder dislocation motion [5]. The maximum hardness at 4 h (401.6 HV) follows the same reasoning described for WQHT, reflecting an optimal balance between bainitic colony refinement, reduced retained austenite fraction, and a favorable carbide distribution for strengthening [30].

The mechanical properties of 5Cr–0.5Mo steel are illustrated in Figure 3 and summarized in Table 3. Three specimens were tested for each condition, and the standard deviation was calculated. Specifically, Figure 3 displays the σ − ε curves obtained in accordance with ASTM E8 tensile test recommendations [46]. All three specimens of the as-received material (Figure 3a) exhibit similar ductile behavior, with slight variations typical of tensile testing. The specimens exhibit significant elongation, consistent with the area under the stress–strain curves, confirming the ductile behavior of 5Cr–0.5Mo steel in the as-received state. Further, all specimens of the N1HA (Figure 3b), N4HA (Figure 3c), and N8HA (Figure 3d) samples exhibit similar behavior, indicating good repeatability of the measurements. Although grain size varies, S _y remains nearly constant, with a slight tendency to decrease as grain size increases (see, Table 3 and Figure 3b–d). Meanwhile, the S _u values of the specimens slightly decrease with increasing austenitization time. Similarly, %δr and %Ar decrease, indicating reduced ductility of 5Cr–0.5Mo steel after NHT. In general, higher S _u values correlate with lower ductility. However, in this case, a slight decrease in ductility is observed, which differs from the expected trend. This observation aligns with the findings of Park et al. [50], who reported increasing S _y and S _u values with decreasing grain size. Furthermore, grain size and S _y are linked through the Hall‒Petch relationship, as detailed by Sadananda et al. [51]. A potential explanation for this observation is the presence of a mixed bainitic and martensitic microstructure, which may introduce a micro-fragility component when cementite plates within the ferritic matrix break, allowing easier deformation of the ferrite matrix as grain size decreases. This deformation would likely increase the likelihood of bainite failure and resulting in reduced mechanical strength and deformation capacity.

Figure 3:

Stress‒strain curves of 5Cr–0.5Mo steel specimens, a) as-received, b) N1HA, c) N4HA, and d) N8HA (S1, S2, and S3 correspond to specimens 1, 2, and 3, respectively).

Table 3:

Mechanical properties of as-received 5Cr–0.5Mo steel and N1HA, N4HA, and N8HA specimens.

Property	As-received	N1HA	N4HA	N8HA
S y (MPa)	186.5 ± 32.3	758.7 ± 38.5	744.4 ± 31.7	739.7 ± 39.7
S u (MPa)	461.1 ± 24.8	1,127.0 ± 5.5	1,025.4 ± 14.5	973.0 ± 7.3
A r (%)	68.9 ± 1.2	48.2 ± 2.7	46.3 ± 0.6	44.6 ± 3.4
δ r (%)	42.6 ± 1.1	18.15 ± 0.4	16.7 ± 0.9	15.4 ± 1.3

3.4 Fracture surface

The effect of the NHT on 5Cr–0.5Mo steel was analyzed using SEM. Specifically, SEM micrographs (Figure 4) of the surfaces of as-received (Figure 4a and b) and N1HA (Figure 4c and d), N4HA (Figure 4e and f), and N8HA (Figure 4g and h) tensile specimens were captured. Figure 4 reveals that %Ar decreases as austenitization time increases, consistent with the values reported in Table 3. In all cases, ductile fractures and coalesced microcavities are observed; however, these features become less pronounced as austenitization time increases.

Figure 4:

Fracture surfaces of the specimens, a) as-received sample at X16, b) as-received sample at X200, c) N1HA at X16, d) N1HA at X2000, e) N4HA at X16, f) N4HA at X2000, g) N8HA at X16, and h) N8HA at X2000.

Figure 5 displays four representative tensile curves for the as-received 5Cr–0.5Mo steel and N1HA, N4HA, and N8HA specimens. This figure highlights the significant influence of NHT on the mechanical properties of 5Cr–0.5Mo steel. The curves reveal a significant reduction in ductility and a notable increase in S _y and S _u following NHT. Notably, S _u follows the trend of N1HA (1 h) > N4HA (4 h) > N8HA (8 h). This indicates that for short austenitization times (≤1 h), S _u increases sharply, reaching a peak near 1 h, following which it gradually decreases with longer austenitization times.

Figure 5:

Stress‒strain curves of 5Cr–0.5Mo steel specimens, as-received, N1HA, N4HA, and N8HA.

Comparative charts for the S _y , S _u , %Ar, and %δr values of the as-received steel, N1HA, N4HA, and N8HA specimens are shown in Figure 6. Figure 6a reveals that the S _y values of the N1HA, N4HA, and N8HA specimens are approximately 306 % (three times larger) that of the as-received specimen. Despite variations in grain size, S _y remains nearly constant across the N1HA, N4HA, and N8HA specimens, with minimal variations of approximately 2 % between 1 h and 4 h and 0.6 % between 4 h and 8 h. This suggests that S _y is insensitive to austenitization times of 1 h or longer, possibly due to larger grain sizes facilitating defect motion within the structure, in agreement with the Hall–Petch relationship [4], [50]. Moreover, this indicates that the NHT process exerts a more pronounced impact on dislocation interactions with the bainitic matrix and cementite plates (acting as short-range obstacles) compared to grain boundaries strengthening, as previously discussed by Bhadeshia [4] and by Sadananda and Vasudevan [51]. Nondestructive characterization studies on ADI further confirm that the mechanical response is strongly governed by the morphology and volume fraction of ausferrite and retained austenite, rather than by grain size alone [52]. Likewise, in bainitic steels such as SA508-3, the presence of cementite particles within the matrix and along grain boundaries has been shown to act as effective pinning obstacles for dislocations, directly influencing fracture behavior [53].

Figure 6:

Mechanical properties of the as-received 5Cr–0.5Mo steel specimen and N1HA, N4HA, and N8HA, a) S _y and S _u , and b) %Ar and %δr values associated with ductility.

The S _u values exhibit a different trend. For the NHT specimens, S _u increases by approximately 144 % (1.5 times higher compared to the as-received specimen) during the first hour of austenitization but decreases by 9 % at 4 h compared to 1 h and by an additional 5 % at 8 h compared to 4 h. This gradual reduction in S _u , accompanied by the consistent S _y trend, with longer austenitization times, also observed in Figure 5, suggests the influence of one or more fracture mechanisms on the material’s strength. This may be explained by the Hall‒Petch relationship, which links grain size with S _y . Figure 6b reveals that %Ar and %δr are inversely proportional to grain size, with higher values observed for the as-received material. The most significant microstructural changes occur during the first hour of austenitization, leading to reduced ductility, increased grain size, and improved S _y and S _u . During this period, %Ar and %δr decrease by 30 % and 57.4 %, respectively.

4 Conclusions

The microstructure and mechanical characteristics of 5Cr–0.5Mo steel were experimentally investigated under varying treatment conditions: i) no treatment; ii) WQHT with 1, 2, 4, 6, and 8 h of austenitization; and iii) NHT following 1, 4, and 8 h of austenitization. The following main conclusions were drawn from this testing plan:

The as-received 5Cr–0.5Mo steel exhibits a microstructure comprising a ferritic matrix with dispersed carbides. After NHT, this microstructure transforms into a mixture of bainite and martensite, achieving a carbon equivalent percentage comparable to that of eutectoid carbon.

S _y and S _u are sensitive to austenitizing times between 0 and 1 h. However, while S _y remains nearly constant for 1–8 h of austenitization, S _u , %Ar, and %δr exhibit an inverse relationship with grain size.

In addition, hardness values showed a nonlinear evolution, with the highest values observed at 4 h of austenitization, both after WQHT (353 HV) and NHT (401.6 HV), indicating an optimal balance between carbide redistribution, retained austenite fraction, and martensitic/bainitic transformation. Fracture surface analysis confirmed the progressive reduction in ductility with increasing austenitization time, as microcavities became less pronounced and fracture morphology evolved from ductile dimples toward features associated with lower toughness.