Influence of gas nitriding on fatigue resistance of maraging steel

International Journal of Fatigue 21 (1999) 163–168

Influence of gas nitriding on fatigue resistance of maraging steel K. Hussain *, A. Tauqir, A. ul Haq, A.Q. Khan Metallurgy Division, Dr. A.Q. Khan Research Laboratories, P.O. Box 502, Rawalpindi, Pakistan Received 18 August 1997; received in revised form 1 September 1998; accepted 1 September 1998

Abstract Marage-350 steel containing 12 and 24 vol% of retained austenite was subjected to gas nitriding at 450°C for 8 h in ⬇ 35% dissociated NH3 gas atmosphere. The depth of the nitride layer decreased from 55 to 35 ␮m as the retained austenite content is doubled. The layer was predominantly composed of Fe4N phase. The surface hardness of the material containing 12 and 24 vol% of austenite after nitriding was improved from 536 to 753 Hv and 470 to 705 Hv, respectively. Fatigue tests were performed on smooth specimens at a stress ratio (R) ⫽ 0.1. It was shown that nitriding played the principal role in the improvement of fatigue strength and sub-surface crack nucleation.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Austenite; Fatigue; Maraging steel; Nitride

1. Introduction The effect of nitriding on the fatigue behaviour of steels has been studied by several researchers. Cowling [1] reported that ion and gas nitriding can significantly improve the fatigue properties of En41B steel. The work of Luan et al. [2] on rotating bending fatigue on different ion-sulfo-carbonitrided steels, in both notched and unnotched specimens, showed an improvement in high cycle fatigue resistance. Han [3] studied the effect of ion implantation on the fatigue life of a Ti-24V alloy and observed the improvement in fatigue life in both low cycle (strain controlled) and high cycle (stress controlled) fatigue. Fatigue cracks usually initiate from the surface of smooth specimens. In case hardened materials the crack initiation usually tends to shift from surface to sub-surface in high cycle fatigue [1,2,4]. This may be due to the increased hardness of the surface layer, resulting in better resistance to cyclic slip. It is therefore easier for cracks to initiate from internal discontinuities or at the interface of the substrate and the hard layer in case hardened materials. Retained austenite has a beneficial effect on the low cycle fatigue strength of steel [5]. At high strain amplitude, retained austenite undergoes strain-assisted trans* Corresponding author. Tel: ⫹ 51 450 734; Fax: ⫹ 51 452 487

formation to martensite. This transformation-induced plasticity increases ductility and strain hardening rates. Therefore, low cycle fatigue life is enhanced by a moderate amount of retained austenite [6]. High cycle fatigue life, however, does not appear to be enhanced by retained austenite. According to the strain approach to fatigue, high cycle fatigue is stress controlled and benefits by microstructures having high elastic limits and yield strength. The authors’ previous work on the nitriding of thermo-mechanically treated maraging steel proved that the surface hardness can be increased from 650 to 900 Hv, depending upon the prior treatment [7]. Farooq et al. [8] reported that retained austenite has a profound effect on the properties of maraging steel. The authors’ work on the effect of retained austenite on nitriding has been accepted for publication, see Ref. [9]. The objective of the present paper is to investigate the fatigue behaviour of nitrided maraging-350 steel containing a different vol% of retained austenite.

2. Materials and experiment The material used for this investigation was 350-maraging steel with a chemical composition (wt%) of 0.005 C, 18 Ni, 3.7 Mo, 12.6 Co and 1.6 Ti. Solution annealed material was heat-treated at different temperatures to induce volume fraction of 12 and 24 vol% austenite. The

0142-1123/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 1 1 2 3 ( 9 8 ) 0 0 0 6 3 - 2

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detail of the heat treatment to induce different amounts of austenite and its effect on the properties is presented in Ref. [8]. The mechanical properties in the solution annealed condition and with 12 and 24 vol% of retained austenite are: (UTS) 1192, 2157, 1917 MPa and (␴0.2%y) 1025, 1860, 1608 MPa, respectively. Polished and clean samples with different volume fractions of austenite were subjected to gas nitriding at 450°C for 8 h in ⬇ 35% dissociated ammonia gas atmosphere. The crosssections of the specimens were polished and etched (at

room temperature) in an aqueous solution of FeCl3 to reveal the depth of nitride layer and the microstructure. Nitride phases were studied by the X-ray diffraction technique. Fatigue tests were performed at R ⫽ 0.1, on three point bend smooth specimens at different stress levels ranging from ␴max ⫽ 1000 to 1900 MPa. Three samples were tested at each stress level. The loading condition and geometry of the samples are shown in Fig. 1. Samples were polished using 1 ␮m diamond paste before nitriding, and no subsequent polish was carried out after nitriding. The tests were executed up to complete fracture of the specimens. Fracture surfaces were studied by scanning electron microscope (SEM) to establish whether the crack was initiated at the external surface or under the nitride layer.

3. Results and discussion The nitride layers had a uniform interface in all the specimens, irrespective of the volume fraction of austenite content. This can be seen in Fig. 2, where the austenite contents are 12 and 24 vol%. A change in hardness from surface to depth is depicted in Fig. 3, along with the hardness of the materials treated at 450°C but not Fig. 1.

Loading condition and geometry of the sample.

Fig. 2. Growth of the nitride layer in the material with 12 and 24% retained austenite.

Fig. 3. Change in hardness with depth from surface to the core.

K. Hussain et al. / International Journal of Fatigue 21 (1999) 163–168

exposed to NH3 atmosphere. The depth of the nitride layer decreased with the increase in austenite content. The case depth with 12 and 24% of austenite was found at 55 and 35 ␮m, respectively. The hardness with depth in Fig. 3 shows that the plateau of higher hardness bends down after some depth and finally reaches the uniform hardness of the matrix. A comparison of the depths etched out in Fig. 2 and the depth–hardness variation in Fig. 3 reveals that the etched-out depth is the depth of the layer where the nitride phase is predominant, as proved in X-ray diffraction studies. The etched-out depth is 40 and 30 ␮m in the samples containing 12 and 24% volume fraction of retained austenite. The band from 40 to 55 ␮m in the former and from 30 to 35 ␮m in the latter are the regions where the hardness values decrease monotonously, approaching the hardness of the matrix. These are probably the regions where nitrogen atoms have occupied interstitial sites in the matrix and caused solid solution hardening. It can further be seen from the Fig. 3 that the peak hardness (hardness at the surface) decreased as the percentage of austenite increased in the material. X-ray diffraction studies show Fe4N phase in the nitride layer. A detailed study of phase formations and their concentration through the case depth with respect to percentage of austenite is presented elsewhere [9].

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Fatigue lifetimes (corresponding to complete fracture) are presented in Table 1. Three samples at each stress level were tested, and their corresponding fatigue life is designated as Nf1, Nf2 and Nf3, respectively. The best fit curves (solid lines) to the average of the fatigue life versus stress is presented in Fig. 4(a,b) along with the actual lifetime of all the samples. Nitrided material with 24 vol% austenite showed better fatigue resistance than the nitrided material containing 12 vol% austenite contents. At high stress levels, the difference in fatigue life in nitrided material containing 24 vol% austenite and unnitrided material with the same percentage of austenite contents is very small. It seems that a thin nitride layer of ⬇ 35 ␮m is not playing any significant role to improve the fatigue resistance at this stress level. However, in the high cycle fatigue region, the nitrided material containing 24 vol% retained austenite showed higher fatigue resistance than un-nitrided material. Curves in Fig. 4(a,b) also show the improvement in fatigue life as the austenite is increased from 12 to 24 vol%. The results of fatigue tests of nitrided and un-nitrided with 12 vol% retained austenite specimens, presented in Fig. 4(b), showed mixed behaviour. It is observed that at high stress levels the fatigue resistance is not improved even after sub-surface crack initiation. The

Table 1 Constant stress amplitude fatigue results

␴max MPa

%␥ ⬘

Fatigue Life, Nf

Initiation

Nf1

Nf2

Nf3

Nf(ave)

24 24 24 24 24 12 12 12 12 12 12

5024 8600 25 010 50 129 1 200 482 2710 6205 8961 11 854 15 347 24 154

6012 8210 25 500 51 256 1 089 569 3717 6249 7778 9679 14 369 20 055

4785 9042 23 125 52 415 1 300 105 3110 4056 8733 10 800 13 825 13 070

5274 8617 24 545 51 266 1 196 719 3179 5503 8490 10 777 14 514 22 426

Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface Surface

24 24 24 24 12 12 12 12 12 12

6125 22 259 231 750 4 255 810 1200 2679 8032 25 264 210 239 1 009 157

5965 24 369 229 810 4 150 910 1650 2150 7891 23 140 215 357 1 208 060

5815 23 890 226 910 4 405 189 1050 3059 9110 24 489 204 700 1 265 180

5968 23 506 229 490 4 270 636 1300 2629 8344 24 297 210 098 1 160 799

Sub-surface Sub-surface Sub-surface Sub-surface Sub-surface Sub-surface Sub-surface Sub-surface Sub-surface Sub-surface

Un-nitrided: 1875 1612 1538 1350 1200 1875 1612 1538 1350 1200 1088 Nitrided: 1875 1612 1538 1350 1875 1612 1538 1350 1200 1088

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Fig. 4.

Constant stress amplitude fatigue tests of nitrided and un-nitrided material with (a) 24 vol% and (b) 12 vol% retained austenite.

nitrided samples tested at ␴max ⫽ 1875 to 1538 MPa (Region A) resulted in a 2.4 to 1.02 times decrease in fatigue life, while the samples tested at ␴max ⫽ 1350 to 1088 MPa (Region B) showed a 1.25 to 52 times increase in fatigue life as compared to un-nitrided material. The reduced fatigue life in Region A could be due to the significant plastic deformation (at higher stresses) of substrate as compared to the nitride layer of relatively high flow stress. This produces a mismatch of stress between the core and the layer and resulted in shorter fatigue life as compared to un-nitrided material. Similar behaviour of nitrided and un-nitrided material is also reported by Qian and Fatemi [10].

Fracture surfaces revealed a strong bond of the nitride layer with the core, as Fig. 5 shows no delamination of the nitride layer. Similarly, defects such as porosity are also not detectable by SEM. A closer look at the crack morphology shows no sharp change in the crack path across the interface of the nitride layer and the matrix. In the present work, fractographic study showed subsurface crack initiation in nitrided materials while unnitrided material showed initiation from the surface, as shown in Fig. 6. Furthermore, no crack initiation site was affected by the applied stress levels. Gualiano and Vergani [11] reported that in fatigue of nitrided 42CrMo4 EN10083, the crack initiation position

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Fig. 5. Fractograph showing strong bond between the layer and the substrate.

is strongly influenced by the nitriding duration and the strain value applied. It is commonly believed that in homogeneous materials the intrusions and extrusions are responsible for crack initiation on the surface [3]. However, in a nitrided specimen, the extrusion and intrusion process is very limited by the surrounding hard material. As a result, the intrusions that act as stress raisers to promote crack nucleation can not be piled up easily, as compared with the free surface crack initiation situation. This can explain the significant increase in fatigue resistance. It can be seen from Fig. 4 and the detail of the fatigue tests of nitrided and un-nitrided materials given in Table 2, that at a stress level of 1875 MPa the improvement in the life of nitrided material with 24 vol% retained austenite is 13.2%, which exponentially increases at lower stresses. Similar fatigue behaviour was observed in the material containing 12 vol% retained austenite in Region B. Based on the differences in fatigue lives the following empirical relationships are developed for the material: with 24 vol% retained austenite: %⌬N ⫽ 1.6 ⫻ 1011e(⫺0.0125␴max) with 12 vol% retained austenite: %⌬N ⫽ 3.2 ⫻ 1010e(⫺0.0143␴max) The above relationships are plotted in Fig. 7, which are in good agreement with the experimentally observed percentage difference in fatigue life (%⌬N).

4. Summary Experimental tests were carried out on nitrided and un-nitrided maraging steel-350, containing 12 and 24 vol% of retained austenite. A decrease in hardness as a function of depth was observed in the nitride layer. The

Fig. 6. Fractographs initiation.

showing

sub-surface

and

surface

crack

hardened nitride zone was 55 and 35 ␮m in material containing 12 and 24 vol% austenite, respectively. Stress control fatigue tests carried out on smooth specimens showed sub-surface crack initiation in nitrided and surface crack initiation in un-nitrided samples, irrespective of the applied stress levels. The improvements are associated with sub-surface crack initiation caused by residual stress distribution in the nitride layer. It was also observed that the stresses in the external nitride layer are not dangerous, even at the highest stress level, which yielded a fatigue life as short as ⬇ 6000 cycles in the material containing 24 vol% retained

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Fig. 7.

Comparison of experimental and predicted percentage difference in fatigue life.

Table 2 Change in average fatigue life of un-nitrided and nitrided material

␴max MPa

%␥ ⬘

Average fatigue life, Nf(ave.) Change in life %⌬N Un-nitrided

1875 1612 1538 1350 1875 1612 1538 1350 1200 1088

24 24 24 24 12 12 12 12 12 12

5274 8617 24 545 51 266 3179 5503 8490 10 777 14 514 22 426

Nitrided 5968 23 506 229 490 4 270 636 1300 2629 8344 24 297 210 098 1 160 799

13.2 172.8 835 8230 ⫺ 59.11 ⫺ 52.22 ⫺ 1.72 125.45 1347.55 5076.13

austenite. However, the material containing 12 vol% retained austenite exhibited different behavior from that containing 24 vol% retained austenite. At higher stress levels (Region A) the nitrided material resulted in a shorter fatigue life as compared to un-nitrided material. Acknowledgements The authors are thankful to M. Nazeer, S.M. Shah and A. Hussain for their help and co-operation.

References

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