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Comparison of the Dislocation Structure of a CrMnN and a CrNi Austenite after Cyclic Deformation
Product and Process Development, voestalpine BÖHLER Edelstahl GmbH & Co KG, 8605 Kapfenberg, Austria
AGD Seibersdorf, University of Leoben, 2444 Seibersdorf, Austria
Österreichisches Gießerei-Institut, University of Leoben, 8700 Leoben, Austria
Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, 8700 Leoben, Austria
Author to whom correspondence should be addressed.
Received: 14 May 2019 / Accepted: 19 June 2019 / Published: 13 July 2019
In the literature, the effects of nitrogen on the strength of austenitic stainless steels as well as on cold deformation are well documented. However, the effect of N on fatigue behaviour is still an open issue, especially when comparing the two alloying concepts for austenitic stainless steels—CrNi and CrMnN—where the microstructures show a different evolution during cyclic deformation. In the present investigation, a representative sample of each alloying concept has been tested in a resonant testing machine at ambient temperature and under stress control single step tests with a stress ratio of 0.05. The following comparative analysis of the microstructures showed a preferred formation of cellular dislocation substructures in the case of the CrNi alloy and distinct planar dislocation glide in the CrMnN steel, also called high nitrogen steel (HNS). The discussion of these findings deals with potential explanations for the dislocation glide mechanism, the role of N on this phenomenon, and the consequences on fatigue behaviour.
austenitic high nitrogen steel (HNS); cold deformation; fatigue
In the last twenty five years, a lot of research has been conducted regarding interstitially alloyed high nitrogen steels (HNS), especially on high nitrogen austenitic steels, which still have not emerged from their status as highly specialised products for niche markets where a combination of strength, toughness, corrosion resistance, and non-magnetic behaviour is required. Those classical applications are medical implants, drill collars, and retaining rings. To achieve the high strength values required for these applications (Rm > 1000 MPa), a further characteristic of that alloying system is taken advantage of: its exceptional work hardening rate. As a consequence, the alloys are often deployed in a work hardened condition. Additionally, it is common requirement for all these applications that the material has to withstand cyclic loads. Therefore, fatigue resistance is an additional important characteristic of these alloys.
In the case of austenitic steels, the arrangement of dislocations during cyclic deformation is of crucial significance to their fatigue behaviour. The first attempts to define this behaviour in alloys with face centered cubic (FCC) microstructure through their dislocation structures independent of the stacking fault energy (SFE) were done by Feltner and Laird [1
] and Lukáš and Klesnil [2
]. They distinguished between wavy slip for high SFE and planar slip for low SFE materials. Wavy slip implies that cross slip of dislocations can occur easily; in the case of planar slip, cross slip is impeded. Figure 1
a shows the “dislocation distribution map” proposed by Lukáš and Klesnil [2
] (from Reference [3
Parallel to this observation, many authors have reported that nitrogen reduces the stacking fault energy in austenitic steels [5
]. The majority of those investigations have been carried out on CrNi-austenitic steels [10
]. However, there have been other studies which described an increase of stacking fault energy with N [13
], or even a non-monotonous dependence [14
]. Moreover, in recent publications it was stated that—in contrast to twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) steels—for the calculation of SFE in austenitic alloys containing N, the existing methods are is still not reliable [15
Therefore, as far as N is concerned, no clear relationship with the stacking fault energy could be established. A different approach to explain the planar dislocation behaviour of austenitic steels is through short range ordering (SRO), as was already mentioned by Douglas et al. [18
] in 1964. Gerold and Karnthaler stated that there might be an overestimation of the influence of SFE on the mobility of dislocations [19
]. They mentioned that SRO leads to a glide plane softening effect in solid solution. SRO has also been discussed by Grujicic et al. [20
] and Gavriljuk and Berns [21
] in connection with high nitrogen austenitic steels, which would explain the planar slip behaviour of high nitrogen austenitic steel and, as a consequence, its high work hardening rate. Based on these findings, Mughrabi suggested a modified version of Figure 1
a where he replaced the stacking fault energy on the y
-axis, indicating that wavy slip is dominating higher in the diagram, and planar slip is preferred towards the bottom of the diagram (Figure 1
Comparing the two alloying concepts for austenitic stainless steels, different mechanisms for dislocation slip can be identified:
The goal of the investigation was to analyse the dislocation slip behaviours of a CrNi alloy and a CrMnN alloy in a cold worked (CW) condition under cyclic loading.
Conceptualization, R.F.; investigation, K.S.-H., M.B., and H.H.; methodology, R.F.; project administration, R.F.; resources, R.F.; supervision, M.K. and R.P.; visualization, R.F.; writing—original draft, R.F.; writing—review and editing, M.K., K.S.-H., and R.P.
This research received no external funding.
Conflicts of Interest
The authors declare no conflict of interest.
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) Dependence of stacking fault energy on cyclic slip mode and fatigue life [3
], with permission from DGM, 2019; (b
) modified diagram proposed by Mughrabi [4
], with permission from Elsevier, 2019.
Drawing of the sample geometry.
Plot of an S-N curve for S31673 at room temperature in the solution annealed and cold worked condition.
Plot of an S-N curve for CrMnN at room temperature in the solution annealed and cold worked condition.
(a) TEM image of S31673, 14% CW taken from unloaded head of fatigue sample; (b) TEM image of CrMnN, 14% CW taken from unloaded head of fatigue sample; (c) TEM image of S31673, 14% CW taken from the gauge length of fatigued sample. (d) TEM image of CrMnN, 14% CW taken from the gauge length of fatigued sample. (e) TEM image of CrMnN, 14% CW taken from the gauge length of fatigued sample. No distinct glide bands are visible, but there is dislocation accumulation in one direction (red arrow). Red ellipses indicate the beginning of cell formation.
Chemical composition of the test grades in wt%.
Mechanical properties of tested grades in solution annealed (SA) and cold worked (CW) condition at room temperature.
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