Introduction

Among high-strength low-alloy steels one of the most promising is steel 30CrMnSi (30KhGSA, AISI 4130, and AISI 5130) [1].

The standard composition of steel 30CrMnSi is given in Table 1. Steel critical points: Ac 1 = 718°C; Ac 3 = 846°C; maximum Ac 1 = 760°C; Ac 3 = 890°C; preferred austenitizing temperature 890°C [2].

Table 1 Steel 30CrMnSi Chemical Composition
Full size table

Repeated tempering of low-alloy steels is rarely performed since their properties may be corrected by single-stage tempering parameters. Double (two-stage) tempering has a weak effect on sintering properties with tensile testing. However, changes in ductility and impact energy with a change in steel composition, tempering temperature and duration are more complicated. Since carbide phase is precipitated during tempering, the effect of double tempering is more strongly expressed for steels containing silicon. There is probably precipitation of ε-carbide from martensite at low temperature, a result of which will be an increase in silicon /carbon ratio in α-solid solution. In this case carbon is more stable in solid solution, and the proportion of carbide phase will be reduced. If steel contains elements retarding martensite decomposition, impact strength may be increased by selecting the temperature for first and second tempering. As a result of this an optimum combination of steel properties and distribution of carbide phase with an acceptable level of stresses within martensite will be achieved [2].

The effect of tempering regimes on steel properties has been studied in [1]. Tempering lasting 3 h was performed at temperatures from 200 to 600°C with air cooling. It was established that after tempering in the range 250 – 400°C impact strength decreased, i.e., steel was inclined towards temper brittleness. In [4] mechanical properties and microstructure were studied for steel 30CrMnSi after austempering with a different cooling rate and subsequent tempering. It was shown in this work that after austenitizing at 900°C austempering at 360°C for 50 min with water cooling and subsequent tempering lasting 1 h at different temperatures there is bainite formation with rounded particles.

The aim of this work is to study the effect of cooling medium after austenitizing and tempering, normalizing, and secondary quenching with tempering, and secondary tempering in order to select optimum heat treatment parameters for preparing high strength properties with respect to elongation and hardness.

Methods of Study

The starting materials were plates with a size of 360 × 280 × 4 mm of steel 30CrMnSi. Tensile test specimens were cut in accordance with ASTM E8M standard. Steel composition from results of spark emission spectroscopy is provided in Table 1. Comparison of standard and experimental steel compositions shows that the phosphorus content in the test specimens is below the established limit, i.e., 0.025 wt.%.

Heat treatment was carried out in a muffle furnace. According to the ASTM E8M standard heat treatment was performed on two specimens for each regime. Then specimens were tensile tested and Rockwell hardness was measured. Heat treatment regimes are provided in Table 2. It is noted that in previous work [5] the heat treatment process was optimized by experimental planning, as a result of which a regime given letter A in Table 2 was selected.

Table 2 Second Stage Optimization Heat Treatment Regimes for Steel 30CrMnSi
Full size table

Impact tests were carried out on specimens 2 mm wide according to the ASTM E23 standard. Three specimens were tested for each condition.

Friction was accomplished on disks 48 mm in diameter and 3 mm thick according to ASTM G99 (ball-on-disk). Test conditions; linear disk rate 0.1 m/sec; friction path 1000 m; vertical load on a specimen 40 N; air atmosphere; room temperature. The accuracy for specimen weight determination was 0.0001 g. The counterbody (ball) was prepared from hardened steel AISI 52100 (ShKh15). The surface of all specimens before testing was treated by polishing paper No. 320 (mesh number). A load of 40 N was selected on the basis of results of testing specimens under loads 5, 10, 20, 30, 40, and 50 N for a friction path of 100 m. Specimen friction coefficients, weight loss, and specific wear rate were determined during tests. The failure surface and wear were studied using a scanning microscope [6].

Shear testing was carried out on sheet specimens 500 μm thick with a size of 20 × 20 mm. Shear rate was 0.002 m/sec, punch diameter was 3 mm, and the original gap between punch and sheet specimens was 15 – 20 μm. Each test was repeated twice.

The failure surface after tensile, shear, and impact testing, and also the friction surface, were studied by means of a scanning electron microscope fitted with an attachment for energy dispersion chemical analysis. Differential thermal and dilatometric analyses were carried out with heating rates of 30 K/min and 20 K/sec respectively.

Results and Discussion

Heat treatment versions are given Table 2 making it possible to determine the effect of its parameters on steel properties: A is statistically optimum heat treatment regime, obtained previously in the first optimization stage [5]; B is evaluation of the effect of cooling medium after tempering; D is repeated (after regime A) quenching with tempering; E is evaluation of the effect of cooling medium after austenitizing; F is single tempering; G is without quenching before tempering; H is quenching without tempering. Results are provided in Table 3 for determination of mechanical properties after heat treatment by these regimes.

Table 3 Mechanical Properties of Steel 30CrMnSi after Second Optimization Stage
Full size table

The microstructure of specimens in the original condition after optimization (series A) is shown in Fig. 1. In the original condition the structure consists of a ferrite-pearlite mixture and pearlite colonies (Fig. 1 a). After heat treatment steel structure is tempered martensite (Fig. 1 b). Results are shown in Fig. 2 for determination of specimen structure of series A in a scanning electron microscope in secondary and back-scattered electrons. In back-scattered electrons the steel structure is uniform without traces of alloying element segregation. In secondary electrons ultrafine acicular martensite is revealed. Spectroscopy with energy dispersion for this phase showed that its composition is in accordance with steel chemical composition (Fig. 3).

Fig. 1
figure 1

Steel 30CrMnSi structure in the original condition (a) and after optimization (b).

Full size image
Fig. 2
figure 2

Steel 30CrMnSi structure after treatment by optimization regime A (scanning electron microscopy): a) in secondary electrons (morphological analysis); b) in back-scattered electrons.

Full size image
Fig. 3
figure 3

Results of energy dispersion analysis for steel 30CrMnSi after heat treatment.

Full size image

The aim of studying specimens of series H is comparison of their microstructure before and after tempering. It was expected that specimens of series H, not tempered, develop brittleness. However, results appear to be unexpected for steel of this type: σr = 1650 MPa with δ = 9%.

Since series F were not given second tempering, comparison of series F and A shows that second tempering increases σr by 55 – 60 MPa, σ0.2 by 40 MPa, and hardness by 2 HRC. Relative elongation and reduction of area decrease by 0.6% and 1.25% respectively.

A failure surface is shown in Fig. 4 after tensile testing for a specimen of series A. Taking account of the degree of reduction in elongation and cross section it may be confirmed that the main steel failure mechanism is ductile, in spite of the fact that within a fracture a mall cleavage section is observed. The greatest part of the surface has a pitted fracture (dimples). This surface is called quasishear [8].

Fig. 4
figure 4

Fracture surface of steel 30CrMnSi series A specimen after tensile testing (scanning electron microscopy): cleavage area within region of circle.

Full size image

Strain curves are shown in Fig. 5 for a specimens of series A tested in shear. For comparison of results of tests in shear and tension over the horizontal axis values are given for so-called normalized displacement, obtained by special calculations [7, 9]. Values of ultimate shear strength for specimens of series A and correlation coefficient for calculating standardized displacement are given in Table 4. Data obtained with a correction factor agree with results of previous studies [7, 9]. The relationship between tensile stress σ and shear stress τ is described by an expression

Fig. 5
figure 5

Shear test curves for two steel 30CrMnSi specimens of series A: τ is shear stress; Δ is nominal displacement.

Full size image
Table 4 Calculated Results for Correction Factor C
Full size table
$$ \upsigma =C\uptau, $$
(1)

where C is dimensionless correction factor. According to data in [7], C = 1.8. In our work it was established that for specimens of series A coefficient C = 1.72 (Table 4). Sections of a specimens surface from series A after shear tests are shown in Fig. 6, where shear bands are seen clearly at high magnification [7].

Fig. 6
figure 6

Failure surface of series A specimen after shear testing at different magnifications (scanning electron microscopy).

Full size image

Results of impact tests for specimens of series A and specimens in an original condition (without heat treatment) are given in Table 5. It is seen that a specimen in the original condition has better toughness but lower tensile strength. Results for scanning electron microscopy are shown in Fig. 7 for specimens of series A after impact tests. The main steel failure mechanism is ductile, and fracture pits (dimples) are clearly seen [10]. In addition, within the center there are elongated areas that may be a result of formation and merging of cavities. This means that cleavage occurred in these areas of a specimen [11].

Table 5 Steel 30CrMnSi Impact Energy in Two Conditions
Full size table
Fig. 7
figure 7

Fracture surface of steel 30CrMnSi series A specimen after impact testing at different magnifications: a) general view; b, c, d) magnified images of areas 1, 2, 3 respectively; A, B) cleavage areas.

Full size image

The following results were obtained with friction tests: specimen and counterbody weight loss 0.0532 and 0.0003 g respectively; average friction coefficient 0.41, friction force 16.4 N; specific wear rate 1.69 mm2/nm. It is apparent that material with higher hardness has greater wear resistance [12], and therefore weight loss for a counterbody is lower. Friction force was calculated from the friction coefficient [12], and specific wear rate calculated from the ratio of weight loss to relative density, force, and friction path [13]. The friction surface of specimen A is shown in Fig. 8. According to results of studies it may be concluded that with friction for a specimen there is simultaneous action of abrasive, adhesive, and oxidizing wear mechanisms. [14]. Formation of dark areas in Fig. 8 may be connected with separation of the surface and accumulation of oxidized wear products [15]. Region 1 in Fig. 8 b and d are areas of abrasive wear. Region 3 consists of particles of wear products (debris) forming flakes as a result of sticking [15]. Also seen in Fig. 8 a are areas of another composition, forming as a result of counterbody material adhesion. Specimen wear is most significant in this region.

Fig. 8
figure 8

Fracture surface of steel 30CrMnSi series A specimen after wear testing (scanning electron microscopy): a) wear track; b) in back-scattered electrons; c) in secondary electrons; d) magnified image of section 2; 1) abrasive wear area; 3) area with wear products.

Full size image

Dilatometric curves are shown in Fig. 9 after the first (480°C, 2 h) and second (430°C, 1 h) tempering. It is seen that during the first 10 min there are significant volumetric changes caused by temper martensite and conversion of low-carbon martensite into ferrite and cementite [17]. The volumetric effects are much stronger after the first tempering. A reduction in hardness after the first and second tempering comprises 13 and 2 HRC respectively.

Fig. 9
figure 9

Steel 30CrMnSi specimen dilatometric curves: a) after first tempering (480°C, 2 h); b) after second tempering (430°C, 1 h).

Full size image

Conclusions

Optimum mechanical properties of steel 30CrMnSi have been obtained after quenching from 800°C (20 min) and double tempering: at 480°C, 2 h, and 430°C, 1 h. Double tempering facilitates an increase in the amount of precipitates and a reduction in stresses within martensite. Impact specimen failure occurs by a ductile-brittle mechanism. After heat treatment by the optimum regime impact energy for specimens is about 10 J, and shear strength is 630 MPa. The main wear mechanism for steel 30CrMnSi in an optimum condition is adhesive. The main volumetric changes of martensite are observed after the first tempering.