Enhancement of Mechanical Properties of AISI304 Stainless Thin Plate by Low-pressure, Repeated Explosive Hardening

Abstract

In order to improve the mechanical properties of AISI304 stainless thin plate, a low-pressure, repeated explosive hardening process was carried out using the explosive treatment technique. Both the residual microstructures and mechanical properties of the treated material were investigated and compared with the undeformed sample. The results show that microhardness, yield strength and ultimate strength of the treated sample remarkably enhanced during a low-pressure, repeated explosive hardening. We propose that the strengthening mechanisms are closely related to the microstructure evolution during the low-pressure, repeated explosive hardening. The corresponding microstructures in the twice hardening samples were dominated by deformation twins, α martensites, with a few deformation bands. The microstructures in the four times hardening samples were characterized by deformation bands and α martensites, where volume fraction of martensite was larger than that in the twice hardening sample.

1. Introduction

AISI304 stainless steel has drawn significant attention in the field of chemical, automobile, aerospace, and nuclear industry because of its excellent corrosion resistance, appearance, and mechanical properties.^1,2) However, with the development of aerospace industry and improvement of manufacturing technologies, the performance of AISI304 stainless steel treated with common strengthening processes has not met the desired requirements in these fields.^3,4,5,6) Therefore, to explore an effective method to remarkably strengthen the AISI304 stainless steel has become a vital issue.

Explosive forming such as explosive welding, explosive drawing, and explosive hardening have been extensively used in various industrial fields due to its high efficiency, energy saving, rapidity, low cost, and high hardness.^7,8) One of the advantages of explosive forming is the extreme hardening effect of processed metals. The previous works^9,10,11) have established that the hardness of treated materials is improved by the microstructural changes arising from the shock waves propagated through the materials. Furthermore, the propagation of a high-intensity shock pulse in the metals increases not only the microhardness of the entire metal but also the yield and tensile strength, so that the material offers more resistance to impact-type wear.

Hence, to improve the mechanical properties of the AISI304 stainless steel plate, several studies have been conducted using explosive hardening methods. Most of these studies have focused on the investigation of relationship between the microstructure and mechanical properties after explosive hardening and shock loading. For example, F. C. Liu and F. C. Zhang et al.^12,13) conducted explosion hardening on pure high Mn steel crossing with shock loaded to 15 GPa. They obtained high Mn steel crossing with hardened layer of 25 mm thickness, suggesting that surface hardening was due to the dislocations and twin strengthening. H. J. Kestenbach et al.¹⁴⁾ investigated the residual microstructure and mechanical response of 3.2 mm thick AISI304 stainless steel plate explosive-loaded to a peak pressure of 10 GPa for different grain sizes. The results showed that both α-martensite and ε martensite occurred in all shock-loaded specimens and became more extensive with decreasing grain size. The strengthening efficiency of shock loading decreased with increasing grain size. K. E. Aeberli et al.¹⁵⁾ explored the effects of explosive shock on the microstructure of 304 stainless steel plate of about 5 mm thickness by explosive loading assembly at the pressure of 25 GPa and time duration of 3 μs. The results showed that hexagonal ε and γ twins occurred in the same grain and that α-martensite particles were formed at the intersection of two γ twins or ε plates. Jerzy Gronostajski et al.¹⁶⁾ prepared 16 mm thick HSLA steel by means of explosive hardening at the pressure of 20 GPa and then compared its properties and structure to cold-rolled HSLA steel. The results showed that the explosively hardened steels display better strength properties than cold-rolled steels for the same degree of deformation. R. Montanari et al.^17,18) conducted repeated explosive deformation on the 5 mm thick 304 stainless steel at the pressure of 27 GPa and pulse durations 15 μs using an explosive loading set-up. Their results proved the formation of α-martensite in the austenitic grains with a pronounced texture and inhomogeneous strain distribution in different crystalline planes. Marc A Meyers¹⁹⁾ carried out explosive shock loading on the 3.2 mm thick 304 stainless steel plate at different temperatures and pressure of 14.8 GPa. He found out that α-martensite was favored during the release portion of the explosive wave and its amount decreased with increasing pulse duration. Murr et al.^20,21) systematically investigated the microstructures and the properties of the shock-loaded AISI 304 stainless steel plates in the pressure ranges from 15–35 GPa. They established simple schematics to explain the connection between plane shock and spherical shock loading in relation to the stress-strain diagram as a paradigm. A high density of the deformation twins and dislocations contribute to the residual hardening. Meanwhile, significant point defects concentration exists. In our previous works,^22,23) we successfully prepared a hardened layer on a 6 mm thick carbon steel plate and pure copper plate using single explosive impact technique. The experimental results indicate that the hardening effects contributed to high-density dislocation walls and dislocation tangles.

Therefore, majority of work has focused on the investigation of microstructures and mechanical properties of AISI 304 stainless thick metal plate (>3 mm) after the single explosive hardening at large shock pressures (10 GPa–30 GPa). However, in the case of thin plates (about 0.5 mm), single explosive hardening at large shock pressures would damage an entire plate. Accordingly, the repeated explosive hardening at a low pressure (<5 GPa) would be appropriate to enhance the mechanical properties of the thin metal plate (about 0.5 mm). However, to the best of our knowledge, there have been limited reports on the effects of repeated explosive hardening at relatively low pressure (<5 GPa) on the residual microstructures and mechanical properties of AISI 304 stainless thin plate. Therefore, this study aims to improve the mechanical properties of AISI 304 stainless thin plate by repeated explosive hardening at a low pressure (<5 GPa). The corresponding microstructures and mechanical properties were systematically investigated to elucidate the relationship between the mechanical property enhancement and microstructure evolution.

2. Experimental

2.1. Sample Preparation

The materials studied here were commercial AISI304 stainless steel plates with the following chemical composition: 0.07% C, 8.04% Ni, 0.01% S, 0.025% P, 1.21% Mn, 0.24% Si, 18.2% Cr, and the balance Fe. The samples were cut into rectangular plates with the size of 300 mm×100 mm×0.5 mm. The explosive hardening process is shown in Fig. 1. The primary components were the flyer plate (AISI304 stainless steel) and the anvil block (45# steel, smooth surface, 100 mm thick). The explosive powders with density and detonating velocity of 900 kg/m³ and 3500 m/s were used for explosive hardening process. Explosive mass with thickness of about 0.015 m were defined as 60% of the amount required by explosive welding. The samples were deformed two and four times, respectively, by explosive flyer plate impact (hereafter, the sample after twice hardening referred to as 2nd-samples and four times hardening referred to as 4th-samples).

Fig. 1.

Explosive hardening process. (Online version in color.)

To determine the pressure of collision between the flyer plate and anvil block during explosive hardening, the velocity of flyer plate, V_p, was calculated first by Deribas’s equation:²⁴⁾   

$$V_P=1.2D\left[\frac{{\left(1+\frac{32}{27}R\right)}^{1/2}-1}{{\left(1+\frac{32}{27}R\right)}^{1/2}+1}\right]$$(1)

where D=3500 m/s is the detonating velocity and R=0.3 is the loading ratio (mass of explosive thickness per unit mass of flyer plate). The velocity of flyer plate, V_p, estimated using Eq. (1) was 319 m/s.

If the collision between flyer plate and anvil block is assumed as symmetric impact, the associated particle velocity of the flyer plate and the anvil block would be equal to one-half of the impact velocity that can be expressed as U_anvil= U_flyer=V_P/2.²⁵⁾ And then the pressure of collision P was calculated by Eq. (2):²⁵⁾   

$$P={\rho }_{flyer}(C_{flyer}+S{U}_{flyer})U_{flyer}$$(2)

where ρ_steel=8000 kg/m³ is density and C_steel=3570 m/s and S=1.92²⁵⁾ are the constants of materials under shock loading. P=5 GPa was flexibly estimated by Eq. (2).

2.2. Characterization

The constitution of phase was monitored using an X-ray diffraction instrument (D/MAX-2500, RIGAKU, Japan) on the impacting plane as shown in the Fig. 2. The Transmission Electron Microscopy (FEI Tecnai G2 F30, USA) and selected-area electron diffraction (SAED) were used to analyze the microstructures of the hardened samples. The microstructures and the distributions of the phase were observed using SEM and EBSD. EBSD data was collected with FEI 430 field-emission scanning electron microscope. The analyzed area was sufficiently large to be a statistical representative of microstructures. The scanning step was 0.3 μm. The HKL Channel 5 software was used to analyze and display the data.

Fig. 2.

Schematic diagram of the sample position.

After the explosive hardening, the reductions of thickness were happened, at the meantime, the surface become uneven. Therefore, the rough surfaces are ground off about 0.05 mm to uniform the surface roughness to measure the mechanical properties. Room temperature hardness was measured using an HXD-1000YMx tester with the test loading of 0.5 N. The tensile tests were performed on a universal testing machine (Instron5500R, Germany) at the room temperature. The samples for tensile testing were cut both from the un-deformed plates and explosive hardening plates using a wire electric discharge machine. The size of the tensile samples was non-proportional and standardized according to the Chinese Standard Metallic Materials-Tensile testing at ambient temperature GB/T 228.1–2010, as shown in Fig. 3. Engineering strain measured by Room temperature extensometer and corresponding strain rate was 10⁻³/s. Three samples were used in each tensile test to determine the tensile strength of each sample.

Fig. 3.

Dimension of the tensile test sample. (Online version in color.)

3. Results and Discussion

3.1. Enhancement of Mechanical Properties

The engineering stress-strain curves for AISI304 stainless steel plates before and after repeated explosive hardening are presented in Fig. 4. The corresponding values of the yield stress, UTS, and elongation on the curves are shown in Table 1. As shown in Fig. 4, the samples were significantly strengthened after repeated explosive hardening. Also, as per Table 1, yield stress and UTS substantially increased with the increasing number of the explosive hardening processes at the expense of small amount of elongation, exhibiting 116% increase in Yield Strength and 27% increase in UTS with only 30% decrease in elongation at maximum. For the two treated samples, the yield strength increment of 4th-sample was 50% higher than that for 2nd-sample while UTS value of 2nd-sample was closed to that for 4th-sample.

Fig. 4.

Engineering stress vs. engineering strain. (Online version in color.)

Table 1. Tensile properties.

Type	Yield Strength (MPa)	Ultimate Tensile Strength UTS (MPa)	Elongation (%)
Undeformed sample	305±10	721±5	65±2
2nd-sample	481±15	853±8	50±4
4th-sample	660±23	896±12	45±5

The microhardness of the undeformed and the hardened sample along the thickness are shown in Fig. 5. The microhardness of the hardened samples was significantly enhanced as compared with that for the undeformed samples. And the hardness of the impacted surfaces of the treated samples was more enhanced as compared to that for the central area throughout the entire thickness, where the values were ranging from a minimum of 300 HV to a maximum of 420 HV. Microhardness increased with the increasing number of explosive hardening. These results can primarily be attributed to the increasing volume fractions of α martensite and crystal lattice defects in the austenite grain during the repeated explosive hardening, which is presented in detail in the following section. To further explain the above strengthening behavior under repeated impacts, the microstructure and martensite transformation were analyzed using XRD, EBSD, TEM and SEM.

Fig. 5.

Microhardness along the thickness. (Online version in color.)

3.2. Microstructure Characterization

3.2.1. XRD

Figure 6 shows the XRD patterns of the undeformed sample and hardened samples. After explosive hardening, compared with the undeformed sample, the intensity of the diffraction peak γ(200) and γ(311) decreased, and inversely that of diffraction peak γ(220) increased with increasing number of explosive hardening processes. Figure 6(b) shows the details of the three regions (1, 2, and 3 in Fig. 6(a)) in which the diffraction peak of α-martensite could be expected. In the region 1, the peak α (110) at 2^θ=44.4⁰ emerge in 2nd- samples and 4th-samples. In the region 2, there exists a diffraction peak α (200) at 2^θ=62.8⁰. In the region 3, diffraction peak α (211) appeared at each stage, the intensity of which increased with the increasing number of explosive hardening processes. We conclude that α-martensite transformation occurred during the repeated explosive hardening process and its volume fraction increased by increasing number of explosive hardening processes. The tendency presented in our results is in good agreement with the previous works.^17,26) However, based on the results of the XRD patterns in this work, the total volume fraction of the martensites was smaller than that for the reported data, which can be attributed to the lower impact pressure (about 5 GPa) and shorter time duration generated by the explosive hardening set-up in this work.

Fig. 6.

XRD patterns before and after repeated explosive hardening (a) XRD patterns of AISI304 stainless steel plate (b), (c), (d) Details for main diffraction peaks of α- martensite. (Online version in color.)

3.2.2. EBSD Analysis

Series of EBSD measurements were conducted to characterize the microstructures of samples before and after repeated explosive hardening. Figures 7(a) and 7(b) show the microstructures of the undeformed sample composing of austenite and a little α-martensite. In the 2nd-sample, as shown in Figs. 7(c) and 7(d), several twin boundaries²⁷⁾ were generated in the austenite grain because of the extreme strain rate (about 10⁵%) and impact pressure (about 5 GPa) in the AISI 304 stainless steel. Meanwhile, several narrow deformation bands and α-martensite lath were formed in the austenite grains. For 4th-samples (Figs. 7(e) and 7(f)), relatively numerous deformation bands and α-martensites were observed. Some martensites which nucleated in the 2nd-samples grew up in the 4th-samples. The volume fractions of α-martensite appeared to increase with an increasing number of explosive hardening processes. These results are in good agreement with the XRD results. Also, as shown in Fig. 7, ε-martensite which usually appears as the intermediate phase during the formation of α-martensite, alternated with α-martensite in a single deformation band.²⁷⁾ By comparing Figs. 7(d) to 7(f), we can observe that twin boundaries in the 2nd-sample became less than those in the 4th-samples. Hence, twin boundaries in the 2nd-sample was likely to evolve into the nucleation sites of the α-martensite in the 4th-samples.²⁸⁾

Fig. 7.

EBSD observations of AISI304 stainless steel plate before and after repeated explosive hardening. (a), (c), and (e) show microstructural morphology and (b), (d), and (f) indicate volume fraction of phases for the undeformed sample, 2nd-sample, and 4th-sample, respectively (red for α-martensite phase, blue for austenite phase, and yellow for ε-martensite phase). (Online version in color.)

3.2.3. TEM Observations

To further reveal the microstructural evolution in the AISI304 stainless steel plates during the repeated explosive hardening, microstructural features of specimens before and after explosive hardening were examined using TEM. As shown in Fig. 8(a), the microstructure of the undeformed sample primarily contained austenite, which is illustrated by the inserted SAED pattern. Figures 8(b) and 8(c) show that single direction deformation bands and its intersections occurred in austenite grain in the 2nd-sample. SAED pattern of the circle marked area in Fig. 8(c) is shown in Fig. 8(d). We can observe mirror symmetry with respect to a common (111) plane which is approximately parallel to the twin plane. Figures 9(a), 9(b) illustrate twin-twin intersections and the nucleation of α-martensites in the 2nd-sample. As shown in Fig. 9(b), embryos of α-martensite were confined in the deformation bands and preferentially nucleate at the intersections of deformation bands. As shown in the Figs. 9(c) and 9(d), SAED pattern of area taken from the circle in Fig. 9(b) reveals that the diffractions were composed of γ-matrix, deformation twins, and α martensite with the zone axis [110] γ//[110]twin//[111]α. This orientation relationship is the well-known kurdjumov-Sloys (K-S) relationship during γ→α transformation in iron alloys.²⁹⁾ It also indicates that being dominated in 2nd-samples, twins and its intersections became the preferred nucleation sites for the α-martensite embryo.

Fig. 8.

TEM micrographs of AISI304 samples before and after repeated explosive hardening (a) Undeformed sample, (b) Single direction deformation twins in 2nd-sample, (c) Intersections of deformation twins in 2nd- sample, (d) The SAED pattern of the area indicated by a circle in (c). (Online version in color.)

Fig. 9.

The nucleation of α martensite in the 2nd-sample (a) micrographs of martensite at intersections of deformation band. (b) Enlarged image of the rectangular block in (a). (c) The SAED pattern of area indicated by the circle in (b). (d) The indexed pattern of (c) and orientation relationship between α and γ. (Online version in color.)

Figure 10 shows the TEM micrographs of the 4th-samples. The 4th-sample contained more embryos of α-martensite than the 2nd-sample (Fig. 10(a)), indicating that nucleation sites of α-martensite increased with the increasing number of the explosive hardening processes. As shown in Fig. 10(b), deformation twins and ε-martensite coexisted in the neighboring grain. The ε-martensite with the zone axis γ [011]//ε[2110], which is another type of deformation induced martensite with hexagonal crystal system, was identified by Fig. 10(d). The SAED pattern of the area denoted by red circle in Fig. 10(b) was composed of γ-matrix and ε-martensite. The absence of the ε-martensite in the 4th-sample is in agreement with the results of the previous studies,^15,29) which suggests that hcp structure would be formed if the slips of stacking faults are superposed on every second γ(111). Another heavily deformed area (Fig. 10(c)) shows that α-martensite and deformation bands were so dense and fragmented that it is impossible to distinguish them from the austenite matrix. Also, these results suggest that morphology of the α-martensite changed from lath-type in the 2nd-sample into a more massive and irregular morphology in the 4th-samples.

Fig. 10.

TEM micrographs of the 4th-samples (a) Micrographs of α-martensite. (b) Micrographs of the twins and ε-martensite (c) Heavily deformed grains. (d) The SAED pattern of the area indicated by the circle in (b). (Online version in color.)

3.3. Strengthening Mechanism of Repeated Explosive Hardening

The microhardness and the tensile properties were enhanced via explosive impact in this work. These results are consistent with previous studies^9,10) and can be attributed to unique residual microstructures in which deformation twins, deformation bands, and α martensite transformation co-existed in the AISI304 stainless steel (see Figs. 7, 8, 9, 10), i.e., the residual microstructures comprised α martensite reinforcing phase and ductile γ-matrices. Therefore the yield stress was described as follows:   

$${\sigma }_y=f_{\gamma }\times {\sigma }_{y,\lambda }+f_{\alpha }\times {\sigma }_{y,\alpha }$$(3)

where σ_y is the yield stress, f_γ is volume fraction of austenite, f_α is volume fraction of α martensite. σ_y,λ is friction stress of impacted austenite grains. According to the previous works^30,31) describing the contribution of deformation twins and grain boundaries towards yield stress in the shock loaded AISI304 stainless steel, we assumed that the twin boundaries and deformation bands were identical to the grain boundaries with regards to strengthening the samples. Therefore σ_y,λ can be described as follows:   

$${\sigma }_{y,\lambda }\text{=}{\sigma }_{0,\lambda }+{\text{k}}_1{D}^{-1/2}+{\text{k}}_2{\lambda }_1{}^{-m}$$(4)

Where σ_0,λ is the friction stress of austenite grains, σ_0,λ=200 MPa,^31,32) D is the average grain size, λ₁ is the twins and deformation bands spacing, k₁=411 MPa·μm^0.5, k₂=200 MPa·μm^{0.5 32,33)} and m=0.5 or 1 related to the twins and deformation bands spacing.

σ_y,α is friction stress of α martensite can be obtained by   

$${\sigma }_{y,\alpha }={\sigma }_{0,\alpha }+{\text{k}}_3{\lambda }_2{}^{-m}$$(5)

Where σ_0,α=220 MPa is the friction stress of α martensites,³⁴⁾ λ₂ is the embryo sizes of the α martensite, k₃=240 MPa·μm^0.5.³⁴⁾

In 2nd-sample, the deformation twins, as shown in Fig. 8, extensively appeared in comparison with the undeformed sample because of explosive loading (about 5 GPa) which greatly exceeded the critical stress for twining. α martensites also occurred in twin-twin intersections. These characteristics were useful for improving the tensile properties. Therefore, we estimated the yield stress of 2nd-sample by formula (3), where f_α is 2% estimated by direct observation on EBSD images.³⁵⁾ In the explosive hardening, grain size increased along with the depth increment from outer surface, as illustrated in Fig. 11. So the average grain size of 32 μm was used to determine the value of D in formula (4). λ₁=0.7 μm and λ₂=0.25 μm were determined from Figs. 8–9. m=0.5 because twin spacing was larger than 150 nm in this study.³⁶⁾ Finally, the yield stress of 2nd-samples was readily calculated using Eq. (3) to be the value of 544 MPa.

Fig. 11.

Microstructure on the thickness after explosive hardening (a) Near the impacted surface (b) 120 μm depth from the impacted surface (c) 240 μm depth from the impacted surface.

Compared to the 2nd-sample, the amount of α martensite and deformation bands in the 4th-sample increased with the number of explosive hardening processes, as shown in Fig. 10. Mechanical properties of the 4th-sample were primarily enhanced by austenite grain boundary, α martensite and deformation bands. Hence, For 4th-samples, f_α is 4% estimated from EBSD observation and value of the D estimated to be 28 μm in formula (4), λ₁=0.3 μm and λ₂=0.15 μm in formula (5) were determined from Fig. 10, m=1,³⁵⁾ Therefore, the yield stress of 4th-samples was readily calculated using Eq. (3) to be approximately 735 MPa.

The calculated results show that contributions of α martensites to the yield stress in both 2nd- and 4th-samples were less than that of deformation twins and deformation bands because of the small volume fractions of α martensites. In addition, the calculated results were 40–60 MPa larger than the results of the tensile tests. These results can be explained by the fact that ε-martensite generated in the deformed grains instead of deformation twins, as shown in Fig. 10(d). Moreover, the ε-martensite formation is known to contribute less towards mechanical properties as it is stress-assisted^37,38) and twins remarkably affected the mechanical properties.

4. Conclusions

The microstructure and mechanical properties of AISI304 stainless steel thin plate deformed by repeated explosive hardening at the low pressure was investigated using XRD, EBSD, and TEM. The conclusions can be summarized as follows.

(1) High mechanical properties of AISI304 stainless steel thin plate were successfully obtained using repeated explosive hardening under low pressure (<5 GPa).

(2) Mechanical properties were monotonously enhanced by increasing the number of explosive hardening processes and the corresponding microstructure changed from austenites to deformation twins, deformation bands, and α-martensites.

(3) Deformation-induced α-martensite was generated in both the 2nd-hardening samples and 4th-samples. Deformation twins dominated in the 2nd-hardening sample and the deformation bands and α-martensite are dominated in the 4th-samples.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No. 51665042).

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