Investigations of Fatigue Damage in a Nitriding Low-Carbon Bainitic Steel for High-Performance Crankshaft

Abstract

In the automotive environment, the need to increase the performance of materials requires extra engineering efforts. The possibility of developing new materials is strategically important. Indeed, alternative solutions in terms of material choice allow designers to optimise their projects and keep competitive production costs. Traditional quenched and tempered steels are usually used for highly stressed components, and possible alternatives could be important competitive opportunities. One possible substitute is using bainitic steels to exploit their economic advantages while maintaining acceptable mechanical performances. This paper explores the fatigue life behaviour of a new low-carbon bainitic steel for applications requiring case hardening treatment obtained by the nitriding process. A high-cycle fatigue (HCF) strength assessment is conducted through a test campaign to compare treated and untreated material. The improvement in fatigue strength is evaluated as well as the study of fracture surfaces, residual stress, and microhardness profiles to assess in detail the effectiveness of the nitriding process. It is found that the nitriding leads to an improvement in fatigue life but not as much as expected because of the low ductile behaviour of this steel, the high speed of stress application added, and the embrittlement of the nitriding treatment, as confirmed through fracture surface analysis.

1. Introduction

Bainitic grade steels are utilised in several industrial applications and automotive powertrain components. Advantages characterise this kind of steel in terms of process time and overall costs compared with tempered and quenched steels. In fact, bainitic grade steel permits high mechanical properties without the time and economic burden of performing numerous heat treatments, such as in tempered and quenched steels. Evaluating the high-cycle fatigue (HCF) performance of martensitic grade steel is crucial for correctly assessing whether such materials should be used in specific high-value-added applications competing against high-strength steel. This information could be used in the materials selection phase [1,2,3] and to identify a better solution in many industrial applications [4]. This is particularly true in contexts where the materials are highly stressed, and the loads are time-dependent. An example of this type of component is the crankshaft for an internal combustion engine.

So dealing with fatigue in highly stressed components, it is interesting, as discussed in the paper by Citti et al. [5], to evaluate the fatigue limit of typical bainitic steel considering not only the untreated material but also the nitriding as a case hardening treatment. The nitriding process allows the development of high fatigue performances necessary for highly stressed mechanical components. Indeed, nitriding treatment for traditional steels allows adding 50% or more to fatigue resistance to the untreated material, as shown by several authors [6,7,8,9,10]. This increase in the fatigue limit is due to compressive residual stresses on the nitrogen diffusion layer below the actual applied stress onto the component itself. This article aims to study a bainitic steel’s behaviour after a traditional gas nitriding process in terms of nitrogen diffusion and fatigue limit increase compared to the untreated material. The method for the investigation of HCF behaviour is rotating bending. The samples were designed and manufactured in line with the machining usually used for crankshafts.

The following paragraphs show:

• Section 2, Materials and Methods, describes the Characterization of the steel in terms of chemical composition, static mechanical properties, heat treatments cycle process, and description of the specimens used for the HCF tests carried out;
• Section 3, Results and Discussion, shows microstructure analysis of nitrided and untreated specimens, the results in terms of the HCF performance of the material, the fracture surface analysis, and the residual stress analysis;
• Section 4, the Conclusion, summarises the results, focusing on the limitations and future developments.

2. Materials and Methods

Low-carbon bainitic steel has been studied in this paper.

Table 1. Chemical composition of the steel used in this study (wt.%).

Composition	wt%
C	0.16
Mn	1.48
Si	0.97
Cr	1.43
S	0.05
Ni	0.18
Mo	0.14
Cu	0.23
Others	0.17 V
Fe	Bal.

shows the chemical composition of the analysed steel obtained using an Arun Artus 8 spectrometer (ARUN Technology Limited, Crawley, UK). The steel is received in vacuum air-remelted bars 80 mm in diameter after a pseudo-forging treatment at 1000 °C for 45 min and air cooling to room temperature (cooling rate range 0.1–0.2 °K/s, room temperature 20 °C). Smaller squared raw bars are cut out from the main bars at an equal distance from the centre. Finally, a stress relief heat treatment at 550 °C for 8 h is carried out. From this condition, specimens are machined, and the nitriding heat treatment in a static gas oven for 72 h at 515 °C is carried out too.

In Figure 1, the heat treatment cycle is summarised, also considering the nitriding phase.

The material’s static mechanical properties (ISO 6892-1) at the core after the heat treatment cycles are collected in

Table 2. Mechanical characteristics of steel after heat treatments cycle.

Characteristic	Value
Proof strength Rp0.2 [MPa]	952
Tensile strength Rm [MPa]	1124
Percentage elongation	18

. The machine utilised for the tensile test is a Galdabini Quasar 200 (Galdabini, Cardano al Campo, Italy) equipped with an extensometer in strain control.

The specimens for the rotating bending fatigue are realised following similar machining phases for an engine crankshaft. For the nitrided specimens, the resistant section is firstly ground and then nitrided. A final grinding (0.05 mm material removal) plus polishing is carried out at the end of the nitriding heat treatment. Identical machining operations are also performed for the non-nitrided specimens (untreated specimens). The final surface roughness values achieved on specimens are comparable with the surface roughness values of a typical crankpin using a Taylor Hobson surface roughness tester (Taylor Hobson, Leicester, UK). Figure 2 shows the characteristics of the finished specimen before the bending test.

Table 3. Roughness parameters measured on the calibrated zone of samples.

Characteristic	Ra [µm]	Rz [µm]	Std. dev. Ra	Std. dev. Rz
Untreated specimens	0.09	0.76	0.016	0.174
Nitrided specimens	0.10	0.79	0.021	0.206

shows the average and standard deviation of the final roughness parameters measured on the specimens after final polishing.

Each specimen is cleaned and degreased in heptane solvent before the tests.

For the evaluation of the nitriding process, hardness profile curves on specimens are carried out. The nitriding depth profiles are realised by interpolating micro Vickers hardness indentations by 0.5 kg load for 15 s using an FM-7 microhardness tester (Future Tech, Tokyo, Japan).

Rotating fatigue tests are performed on a single-point bending test machine at a frequency of 50 Hz in air. Figure 3 shows the machine’s scheme and the specimen’s clamping system.

The staircase method (for more detail on the calculation, see [11,12]) is used to identify the fatigue limit at 10⁷ cycles at the constant step of load variation (25 MPa). There is no observation of heating effects on the specimens during fatigue testing, and pure rotating bending conditions are correctly maintained.

The fracture surfaces of the specimens are examined under stereoscopic using Olympus BX51M (Olympus soft imaging solutions GmbH, Münster, Germany). The morphology of the fractures is analysed by Scanning Electron Microscope (SEM) Zeiss Sigma 300 VP (Zeiss, Oberkochen, Germany, 15 KeV, 2.25 A) to evaluate the crack nucleation and fatigue development. The residual stress analysis is made using an Xstress G3 diffractometer (Stresstech, Jyväskylä, Finland) collimator with a diameter of 2 mm based on the d vs. sin² Ψ method.

3. Results and Discussion

3.1. Microstructure Analysis

Metallographic cross-sections are performed both for the nitrided and the untreated specimens. The microstructure of the material is analysed after etching by Nital 2%. Figure 4a,b show such microstructures at the core and nearby external surfaces of a nitrided specimen without the final grinding and polishing phase. Figure 4c shows the compound layer developed in this steel. The average thickness of this layer is about 13 µm. Some nitriding lamellae developing from the compound layer are detected, and a not-closed net is formed at grain boundaries. Figure 4d shows the microstructure of the untreated specimen, which appears identical to the nitrided specimen microstructure at the core.

As known, the gas nitriding process generally performed between 500–580 °C generates a compound layer, also called the “white layer”, of intermetallic compounds (nitrides and carbonitrides: γ_Fe₄N phase or ε_-Fe₂N phase) [13,14]. This layer is very brittle, and its debris could potentially interpose between mechanical components such as crank pins and bearings, generating failure. So in such applications, it is removed.

Typically, the nitrogen diffuses in this steel, generating a modified structure or “diffusion zone” for a depth of 0.1–0.5 mm. This region consists of stable nitrides generated by the thermochemical reaction of nitrogen with steel.

The main advantage in terms of fatigue resistance, in particular in bending stressed applications, given by the nitriding process consists of generating a compressive state below the surface because of the diffusion of nitrogen and the lattice deformation.

Figure 5 shows two curves of the nitriding depth profiles of the specimens. Two transversal sections by specimen are analysed. The first is in the ground and polished section, and the second is in a raw part without final grinding and polishing operations. The standard deviations of the measurements by every indentation are also added to the graph. The two curves show a constant shift of about 0,05 mm due to the final grinding and polishing.

Four reference depths of the nitriding case have been evaluated regarding the nitriding heat treatment. They are set at 525 Vickers, core plus 100 Vickers, core plus 50 Vickers, and core plus 10% core hardness Vickers.

Table 4. Core hardness and nitriding depth measurements at various references.

Headings	Not Ground Zone	Ground and Polished Zone
Core HV	375	375
Depth 525 HV	0.41 mm	0.36 mm
Depth core + 100 HV	0.43 mm	0.39 mm
Depth core + 50 HV	0.48 mm	0.43 mm
Depth core + 10% hardness HV	0.50 mm	0.46 mm

reports the core hardness and depth values at the various reference points. The nitriding depth measured could be compared with traditional nitrided quenched and tempered steels [15].

3.2. HCF

The staircase method is carried out using 20 specimens to evaluate the fatigue limit. In detail, the limits at the 10th, 50th, and 90th percentile of survival are computed.

Table 5. Fatigue limits reached by untreated specimens after staircase fatigue test.

Heading	10th of Survival	50th of Survival	90th of Survival
Fatigue limit [MPa]	652	626	601

shows the limits obtained through the staircase method.

Figure 6 shows the complete staircase with each single test result. The white circles represent runout cases (number of cycles > 10⁷ cycles), while the black circles represent the failure cases.

Table 6. Stress and cycles by each test of the staircase of the untreated specimens.

Test n°	Stress [MPa]	Cycles
1	650	6.079 × 105
2	625	4.367 × 105
3	600	runout
4	625	2.358 × 106
5	600	Runout
6	625	2.972 × 106
7	600	runout
8	625	runout
9	650	1.381 × 105
10	625	runout
11	650	runout
12	675	3.619 × 105
13	650	3.430 × 106
14	625	runout
15	650	5.690 × 105
16	625	3.692 × 105
17	600	runout
18	625	5.238 × 105
19	600	runout
20	625	9.911 × 105

details the number of failure cycles and the bending stress values applied for each specimen. Notice that the maximum number of cycles counted with a failure of the specimen is 3.4 × 10⁶; above, only runouts of the specimens are collected, enhancing the knee for the S/N curve of the steel. The ratio of the Rm value with the fatigue limit at the 50th percentile is above 50% (56%). This value is in line with high-performance quenched and tempered steel families reported by several studies [16,17].

About the staircase performed onto the nitrided specimens, the fatigue limit at 10⁷ cycles for the 50th percentile survival is at 706 MPa, while for the 10th and 90th percentiles, the values are 723 MPa and 690 MPa, respectively (

Table 7. Fatigue limits reached by nitrided specimens after staircase fatigue test.

Heading	10th of Survival	50th of Survival	90th of Survival
Fatigue limit [MPa]	723	706	690

). The increase in the fatigue limit is about 13%. This value is relatively low if compared with other quenched and tempered steels after the nitriding heat treatment, which can increase their fatigue limit much more compared with untreated conditions [18,19,20]. It is also possible to notice a certain level of data scattering in the presence of fatigue life below 2 × 10⁷ cycles, as suggested by Gui et al. [21].

Figure 7 shows the results of each nitrided specimen tested.

Table 8. Stress and cycles by each test of the staircase of the nitrided specimens.

Test n°	Stress [MPa]	Cycles
1	850	1.805 × 106
2	825	1.780 × 106
3	800	1.397 × 106
4	775	3.995 × 106
5	750	6.865 × 106
6	725	3.053 × 106
7	700	7.369 × 106
8	675	runout
9	700	runout
10	725	2.316 × 106
11	700	runout
12	725	5.668 × 106
13	700	runout
14	725	1.793 × 106
15	700	7.310 × 106
16	675	5.359 × 106

details the number of cycles before the specimen fails in the various tests performed.

Notice in Figure 8 that in several cases, the number of cycles measured at the failure of the specimens is between 4 × 10⁶ and 10⁷ cycles, while no more than 3.4 × 10⁶ cycles with failure are detected for the untreated specimens. Therefore, the knee of the S/N curve for the nitrided specimens is shifted at a higher number of cycles than the untreated steel one. This phenomenon of later initiation of fatigue cracks could be due to nitriding, as suggested by Terent’ev et al. [19].

3.3. Fracture Surface Analysis

The untreated specimens follow typical fatigue development from external surface nucleation to the core. Nucleation in these test specimens occurs from the outer surface and without the presence of any inclusions, as evidenced by specific EDS analysis. In Figure 9a, taken from the stereoscopic instrument, it is possible to assess the development of the fatigue and the final fracture of a specimen. About 30% of the resistant section fails by fatigue propagation. Figure 9b shows a band of crash-fractured material with brittle morphology identical to the crash-fractured heart zone in the transition zone between the fatigue propagation zone and the crash-fractured zone. (detailed in Figure 9c). This phenomenon always occurs near the separation of fatigue and crash propagation.

Moreover,

Table 9. Stress and fatigue propagation by each test of the staircase of the nitrided specimens.

Test n°	Stress [MPa]	Fatigue Propagation [mm]
1	650	1.886
2	625	1.887
4	625	1.794
6	625	1.872
9	650	2.010
13	650	1.982
15	650	1.888
16	625	1.714
18	625	1.705
20	625	2.262
13	650	1.982
15	650	1.888
16	625	1.714

reported the fatigue propagation values for all the broken specimens.

The morphology of the sudden fracture at the SEM reveals a brittle behaviour of the material with quasi-cleavage fracture (Figure 10a) that contrasts with the tensile test performed (Figure 10b). Indeed, specimens showed high elongations and ductility at the fracture surface with dimples (some brittle cracks are detected at grain boundaries). The different speeds of solicitations could justify the different behaviour. In other words, solicitations at high speed, such as the bending fatigue test carried out in this study, can completely change the behaviour of the steel passing from a ductile to a brittle one.

About nitrided specimens, subsurface nucleation is detected. Figure 11 shows examples of the fracture analysed by stereoscopic instrument (Figure 11a,b) and with SEM (Figure 11c,d), with the typical fish-eye fatigue propagation [7,22]. In all tests, the fish-eye area appears circular and white, with the crack nucleation site as a dark area in the centre. This dark area is known as the “Optically Dark Area” (ODA) by Murakami et al. [23] or as the “fine granular area” (FGA) by Sakai et al. [24] and “Granular-Bright-Facet” (GBF) by Shiozawa et al. [25]. Notice that the fatigue propagation is limited to a small portion of the resistant section, about 16%.

Matrix nucleation and fatigue propagation, observed by the SEM analysis (Figure 11c,d), are similar to the phenomena defined as supergrain nucleation by Zhou et al. [17] and Huang et al. [22]. Differently from some other authors’ studies as Zhang et al. [26], Murakami et al. [27], and Bell et al. [15], the nucleation cause cannot be associated with inclusion in the specimens analysed. A confirmation of this is obtained through the EDS analysis of the fish-eye nucleation zone, which did not detect any traces of elements not common to steel (Figure 12).

The nucleation depths from the external surface have been measured by SEM in each broken specimen. These values are used to compute the equivalent applied stresses at the nucleation point by stress distribution of bending along the cross sections of specimens.

Table 10. Fatigue nucleation depth and equivalent stress evaluated at the nucleating point.

Test n°	Fatigue Nucleation Depth [mm]	Equivalent Stress at the Nucleation [MPa]
1	0.748	618
2	0.532	664
3	0.492	656
4	0.709	574
5	0.574	593
6	0.457	604
7	0.533	564
10	0.580	571
12	0.650	553
14	0.780	519
15	0.544	561
16	0.647	516

reports the values of these depths measured by each test and the computed values of stresses acting on the nucleation points.

First of all, must be highlighted the fact that the nucleation depths measured are just above the case/core material transition, where the compressive stresses convert to tensile stresses [7,17]. Secondly, the average of the equivalent stresses is 583 MPa, close to the fatigue limit measured by the staircase test for untreated specimens. This justifies the matrix breakage at the subsurface for all specimens.

The SEM analysis for the nitrided specimens reveals a brittle behaviour at the core of the material (Figure 13a), as seen on the untreated specimens. Moreover, a transgranular fracture for the nitriding zone with no fatigue propagation onto the nitriding case is detected nearby and far away from the nucleation point (Figure 13b,c).

With regard to the crack advance zone, no morphological differences are found with what is seen in the non-nitrided specimens. A detail of the crack advance is shown in Figure 13d. It is possible to note the fish-eye zone at the bottom delimited by the white dashed line at the top the fatigue propagation with similar morphology to that already seen in the non-nitrided samples, and at the left and right limits of the image, the zones characterised by the crash fracture in the core and the nitrided layer, respectively (red lines).

The so brittle behaviour of the bainitic material obtained by the tests justifies that fatigue propagation is limited. Indeed, no plasticity is allowed, so the initial crack as nucleates determines a rapid fatigue development.

The sizes of the fish-eye zones have been evaluated for the nitriding tested specimens.

Table 11. Fish-eye area evaluation.

Test n°	Fish-Eye Area [mm2]
1	0.65
2	0.87
3	0.74
4	0.84
5	1.16
6	1.35
7	1.42
8	1.40
9	1.47
10	1.69

shows these values obtained.

Using data in Table 11, it is possible to evaluate the correlation between the fish-eye area, the number of cycles (Figure 14), and the test loads (Figure 15).

In particular, the correlation between the fish-eye area and the number of cycles seems to follow a nonlinear positive trend (Spearman Coefficient R = 0.806, p-Value < 0.01), and the relationship between the fish-eye area and the test load is a negative linear trend (Spearman Coefficient R = −0.952, p-Value < 0.01).

The subsurface crack initiation is probably caused by reaching the fatigue fracture limit of the bainitic matrix, which led to the formation of the facetted initiation planes, as confirmed by Figure 16. This feature is interesting because, although the level of nonmetallic inclusions present in the material is higher than the hardened reference, this did not affect the outcome of the tests.

Lastly, the nitrided layer acts as a barrier for the crack initiation by moving it under the surface and as a limit for the propagation phase. In fact, fatigue does not progress in the nitrided layer, which breaks apart as soon as the fatigue limit is exceeded (Figure 17).

3.4. Residual Stress Analysis

The residual stresses in the nitriding zone have been evaluated through several measurements taken at different depth levels (Figure 18). A comparison is also made between nitrided and untreated specimens’ stresses on the external surfaces. In this case, the analysis has been made in longitudinal and transversal directions in the notched area of polished specimens (Figure 19).

Results confirm an increase in the residual stresses due to the nitriding diffusion process and machining operations. It is relevant to underline that the non-nitrided material on the external polished surface also shows a compression state. This is due to the heat treatment process (quench and temper) and machining operations performed after it (grinding and polishing). The main effect of compression residual stresses is clearly shown in rotating bending fatigue nitrided specimens where subsurface nucleations appear, while this phenomenon does not appear in non-nitrided specimens.

4. Conclusions

The steel analysed is a bainitic grade steel with high mechanical characteristics and high potential as an alternative material to quenched and tempered steels currently used for high-performance automotive engines, particularly in producing crankshafts. The research aims to characterise the steel’s fatigue resistance considering nitriding’s effect. The untreated material has excellent fatigue results considering its mechanical properties (>50% of Rm). Moreover, the fatigue crack propagates about 30% of the resistant section.

The nitriding applied to increase the fatigue property of the steel leads to good results in realising the nitrided layer, but the expected values of fatigue obtained are not so much improved. In fact, the fatigue limit increase compared to the untreated specimen is only 13% higher. The reason for such a low increase in fatigue limit could be found in the low ductile behaviour of this steel, the high speed of stress application added, and the embrittlement of the nitriding treatment, as confirmed through fracture surface analysis.

Further investigations must be performed to increase the steel’s ductile behaviour by using different heat treatments and going further in understanding the nucleation and propagation of cracks in bainitic materials using highly notched specimens to force the nucleation and propagation outside on the external surface.