The effect of nitrogen and inclusions on fracture and fatigue of steel.

The dynamic strain ageing behaviour of seven heats of C-Mn steel and an interstitial free steel has been examined from 20°C to 350°C, using slow strain rate tests (SSRT's) at strain rates of 2 x 10[-6] s[-1] and 2.67 x 10[-6] s[-1]. The steels were chemically analysed for the key elements, and additional specialised testing was carried out using internal friction to determine the free nitrogen and carbon content. The effects of temperature on the mechanical properties were investigated in detail. In agreement with previous studies, work hardening peaks, minima in ductility, and negative strain rate dependences of the flow stress were observed between 100°C and 350°C. The positions of which depended on the free nitrogen content and strain rate. It was found that the DSA strengthening due to interstitials was found to be proportional to the log of the free carbon plus free nitrogen.An investigation has been undertaken to examine the ductility and J[1c], dJ/da response of a range of experimental steels containing a range of carbon, nitrogen and sulphur values on 25 mm CT specimens. The tests were carried out at 20°C and 200°C, in the so called blue - brittle range. It was found that the crack initiation resistance J[1c] and the crack growth resistance dJ/da in the dynamic strain ageing region were 30% - 40% lower than at room temperature. Increasing the sulphur content resulted in a 5% -10% reduction of these values. With a further reduction of 8% of these values in a 8 ppm DOW environment at 200°C. Environmental testing was done in a chamber specifically designed for the purpose. It was also found that a linear relationship approximates the relationship between the integral of the J vs Qa curve and the Charpy upper shelf energy.From Charpy tests carried out it was found that the Impact Transition Temperature is proportional to the square root of the free nitrogen content, and that a reciprocal type relationship exists between the Charpy upper shelf energy and volume fraction of inclusions. In agreement with previous studies.Fatigue testing was done at 150°C in air on 11.85 mm thickness specimens. Post test examination of fracture surfaces was carried out using scanning electron microscopy. It is shown that increased levels of free nitrogen and carbon reduced fatigue crack growth rates. This is also confirmed by reduced striation spacing. The effect of interstitials on da/dN is shown to be related to increased yield strength.A modified Ashby model is presented which enables engineering evaluations of the effect of volume fraction of MnS inclusions (V[f]), and free interstitial content (N[f]) on the strain to fracture from 20°C to 300°C. When the model was compared to two sets of published data it was found that, within an error bar tolerance of +/-20%, that 66% and 75% of the data points fell within them.