Strength of 10CR-N martensitic steels

lOCr stainless steel has been employed to examine the effect of nitrogen on microstructure and strength. Applying Solid state gaseous nitrogenising treatments a whole range of nitrogen martensite structures containing up to 0.45 wt% were obtained. It was found that a linear relationship exists between strength and nitrogen content in precipitate free martensitic structures. Yield strength increased from 705 to 1295 MPa for nitrogen free base material and alloys with 0.35 wt%N respectively. Pronounce secondary hardening was observed at a tempering temperature of 500°C. A linear relationship was also observed between the lattice parameter and nitrogen concentration in these alloys. A model for mechanical behaviour is presented. MATERIAL AND EXPERIMENTS A commercial alloy was selected to examine the effect of nitrogen alloying in martensitic structures. The material examined was 33% cold rolled strip with 0.65 mrn thickness and chemical composition of 0.016C, 0.002SI 0.38Si, 9.3CrI 0.61M0, 0.95NiI 0.68MnI 0.29V. By solid state nitrogenising and solution treatment at 1200°C nitrogen concentrations ranging from zero to 0.45 wt% were obtained. Tensile test samples with gauge length of 20mm were punched from these strips. Tensile tests were carried out at temperatures from ambient up to 500°C. RESULTS AND DISCUSSION The Yield strength increases linearly with increasing nitrogen content (Figure 1) for steels cooled from solution treatment temperature. It is shown that this tempexature is high enough to dissolve all nitrides [I], and upon cooling all nitrogen will be in solid solution in the lattice. Pronounced secondary hardening behaviour is observed by addition of vanadium and similar ferrite formers to 9-12% Cr nitrogen alloyed materials [1,2,3] and the results of hot strength tests shown in Figure 2 confirm this effect. Alloys tempered at 500°C retain the asquenched strength throughout the test temperature range. This is because of the locking effect of coherent precipitates and dislocation interaction. Lattice parameter measurement for materials in the as quenched condition and tempered at 500°C is presented in Figure 3. Both materials show a linear increase in lattice parameter by increasing nitrogen, with values of 0.0005 nm/at% N as quenched Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993712 98 JOURNAL DE PHYSIQUE IV and 0.0003 nm/at% N after 500°C tempering. Thus in both conditions there is a contribution to strength by nitrogen in solid solution which is a maximum in the as quenched state and is combined with precipitates after 500" C tempering. TEM observation of dislocation networks and electron diffraction patterns [4] show that the strengthening mechanisms in these cases are governed by different mechanisms. The dislocation glide mode in bcc metals is governed to a large extent by the relative mobilities of screw and non-screw (edge) dislocations. It is important to note that, whereas unstable dislocation propagation and multiplication can be facilitated at T>T, (as in fcc metals), these processes are strongly impeded at T200 MPa, independent of interstitial content.The base nitrogen free martensite has a yield strength of 700 MPaand the above parameters add to the strength giving the finalstrength of about 1500 MPa, from which it can be seen that thecontribution from a i in t is about 300 MPa. Quantitatively this innot very much more than the other terms, however it is also clearthat the contribution of these other parameters also dependindirectly on interstitial content. For example in Figure 3, thelinear dependence of lattice parameter vs nitrogen content of theas-quenched material shows a similar lattice dilation withnitrogen content as given in the literature [9]. Thus for thematerial tempered at 500°C which has a smaller apparent dilationthis can be interpreted as a fixed proportion of the nitrogen inprecipitated form at each nitrogen level and from the differencein slope given above 30/53= 60% of the nitrogen remains insolution and 40% is precipitated. The net effect on roomtemperature testing is that the latter case, with someprecipitation, shows a higher strength than as-quenched (Figure 4)for all tempering temperatures up to 500' C, above which coarseningof precipitation and recovery cause a strength decrease. Itshould be noted in Figure 2 however that at test temperaturesabove room temperature both as-quenched and 500°C temper behavesimilarly, since the duration of the test causes someprecipitation in the as quenched alloy. REFERENCES1. F. Bahrami, K. Laing and A. Hendry; in Proc. Conf. onMaterials for Advanced Technology Applications", 4-6,1991,Limerick, Edrs. M. Buggy and S. Hampshire, Trans. Tech.Pub., Vol. 72-74, 153, 1992.2. J. Foct, f. Vanderschaeve and R. Tailard; Proc. Con£.,4.Warmebehandlungstagung, March 1991, Chemnitz, Vol 15-1, 1991.3. Berns H. and Krafft F.; in" High nitrogen steels", proc. ofHNS88, J. Foct and A. Hendry Eds., Inst. of Metals,London,169, 1989.4. F. Bahrami; PhD Thesis, University of Strathclyde, 1993.5. W.C. Leslie; "The Physical Metallurgy of Steels", McGrawHill, 1981.6.P.J. Uggowitzer and G. Stein; Proc. Conf., 1-3 Sep., 1987,Chicago, Illinoise, USA, R. Viswanathan and R.I. Jaffee edrs.,ASM, 181, 1987.7. B.R. Anthamatten et al; Conf. Proc. HNS 90, Stein G. andWitulski H. Eds., Dusseldorf, 436, 1990.8. R.W.K. Honeycomb; "Steels, Microstructure and Properties",Arnold pub., 1981.9. A. Krawitz and R. Sinclair; Phil-Mag., 31, 697, 1975. JOURNAL DE PHYSIQUE IV Figure1 Tensile atrength Va nitrogen Content i n a l loy A, solution treated a t 1200.C.Tesi Temperature [ oC ] Pigure 2 Yield strength Vs t e s t teapeeature i n a l l o y A, 0.287000*I0.00.4 0.81.21.62.0Nitrogen ' [ aim% ].w at.