Effect of nitrogen content on dynamic strain ageing behaviour of type 316LN austenitic stainless steel during tensile deformation

Abstract

The effect of nitrogen content on the dynamic strain ageing (DSA) behaviour of type 316LN austenitic stainless steel has been studied. The nitrogen content was varied from 0·07 to 0·22 wt-%. The tensile tests were carried out over a temperature range of 300–1123 K and at three strain rates in the range 3×10^?3–3×10^?5 s^?1. Serration was observed in the load elongation curves in the intermediate test temperature range and has been considered due to DSA phenomenon. The critical strain to onset of serrated flow increased with increase in nitrogen content and strain rate. The temperature for onset of DSA and the temperature of disappearance of DSA were found to increase with the increase in nitrogen content. The variations in tensile strength and work hardening rate of the steel with temperature exhibit peak values in the intermediate temperature range and have been attributed due to DSA phenomenon. The activation energy for DSA, estimated based on the temperature and strain rate dependences of the strain to onset of serrated flow, was found to increase from 111 to 218 kJ mol^?1 with the increase in nitrogen content from 0·07 to 0·22 wt-% and the increase has been attributed to the possible enhanced interaction of the DSA causing interstitial nitrogen with substitutional chromium.     

Tensile stress–strain and work hardening behaviour of 316LN austenitic stainless steel

Abstract

The true stress (σ)–true plastic strain (?) data of a type 316LN austenitic stainless steel tested at nominal strain rates in the range 3×10^-5–3×10^-3 s^-1 and temperatures of 300–1123 K were analysed in terms of flow relationships proposed by Hollomon, Ludwik, Swift, Voce, and Ludwigson. The applicability of the particular flow relationship is discussed in terms of ‘complete’ and ‘applicable’ range fits of the experimental σ–? data. At all strain rates, in the case of the complete range fit, the Ludwigson equation followed the stress–strain data most closely at 300 K, while in the temperature range 523–773 K, the flow behaviour was described equally well by both the Ludwigson and Voce equations. In the temperature range 823–1023 K, the Voce equation described the flow behaviour most accurately in the case of the complete range fit of σ–? data at all strain rates. The analysis of σ–? data employing the Ludwigson equation in the applicable range fit covering low and intermediate strains, and the Hollomon equation at high strains provided close simulation of the observed flow behaviour in the temperature range 823–1023 K. At high temperatures of 1073 and 1123 K, the Ludwigson equation reduces to the Hollomon equation. The variations in different flow parameters of the Ludwigson and Voce equations with temperature and strain rate exhibited anomalous behaviour at intermediate temperatures because of dynamic strain aging.     

Influence of nitrogen on tensile properties of 316LN SS

Abstract

The influence of nitrogen on tensile properties of 316L stainless steels has been studied for nitrogen levels of 0·07, 0·11, 0·14 and 0·22 wt-%. Tensile tests have been carried out at several temperatures in the range 300–1123 K. Nitrogen was found to be beneficial for tensile strength at all the test temperatures. Yield strength and ultimate tensile strength were found to increase linearly with increase in nitrogen content at all the test temperatures. Tensile ductility showed a non-monotonic variation with nitrogen content and test temperature. Equations have been developed to predict yield strength and ultimate tensile strength of 316L stainless steel as a function of nitrogen content and tensile test temperature.     

Temperature and strain rate effect on tensile properties of 9Cr–1·8W–0·5Mo–VNb steel

Abstract

Tensile tests have been carried out on 9Cr–1·8W–0·5Mo–VNb steel (grade 92) over wide ranges of temperature (300–923 K) and strain rate (3×10^?3–3×10^?5 s^?1). The tensile strength of the steel decreased slowly with temperature at relatively lower temperature range, whereas rapidly in the higher temperature range with a plateau in the intermediate temperature range. The decrease in strain rate decreased the tensile strength of the steel both at lower and higher temperature ranges. Elongation to fracture and reduction in area increased with increase in temperatures and decrease in strain rate at higher temperature regime with a plateau in the intermediate temperature regime. The ductile mode of tensile failure has been observed in the investigated temperatures and strain rates. The plateau in the variation of tensile strength with temperature, the negative strain rate sensitivity of tensile strength and minimum in ductility of the steel in the intermediate temperature range are considered as a consequence of dynamic strain ageing. The rapid decrease in tensile strengths and increase in ductility at high temperatures have been attributed to the dynamic recovery.     

Hot deformation behaviour of precipitation hardening stainless steel

Abstract

The behaviour of 17-4 precipitation hardening (PH) stainless steel was studied using the hot compression test at temperatures of 950–1150°C with strain rates of 0·001–10 s^?1. The stress–strain curves were plotted by considering the effect of friction. The work hardening rate versus stress curves were used to reveal whether or not dynamic recrystallisation (DRX) occurred. Using the constitutive equations, the activation energy of hot working for 17-4 PH stainless steel was determined as 337 kJ mol^?1. The effect of Zener–Hollomon parameter Z on the peak stress and strain was studied using the power law relation. The normalised critical stress and strain for initiation of DRX were found to be 0·89 and 0·47 respectively. Moreover, these behaviours were compared to other steels.     

Thermomechanical properties of iron: viscoplasticity of ferrite and of austenite—ferrite mixtures

The available data on secondary creep in creep tests and on maximum stress in hot deformation tests obtained for δ and α ferrite have been reviewed and expressed in the form of the equation ? = K(σ/E)^P_c sinh(Vσ/RT) exp(—Q_c/RT) with p_c = 2·5, V = 0·0013 m³ mol^?1, and K = 2·6 × 10¹⁷ s^?1, E and Q_c being temperature dependent functions. The approximate range of validity is Z = ? exp (Q_c/RT) ≈ 10¹—10¹¹ s^?1. Together with the corresponding equation for austenite the data can be used to predict the viscoplastic behaviour of austenite—ferrite mixtures.

MST/1538     

Serrated yielding in Nb–V dual phase steel

Abstract

The characteristics of serrated yielding (the Portevin–Le Chatelier effect) in a Nb–V dual phase steel have been studied in the temperature range 85–210°C at strain rates between 1·2 × 10^?5 and 1·2 × 10^?2 s^?1. Serrated yielding was found to initiate only after a critical strain ?_c was reached. The strain between two successive serrations ??_s increases almost linearly with strain, while the stress drop ?σ_c increases with strain up to ?σ_max, then decreases. The exponent β in the mobile dislocation density–plastic strain relationship (ρ_m= ?^β) is 1·09 in the temperature range 85–140°C and 1·34 in the temperature range 140–210°C. The results also indicate that in the same temperature ranges there are two values of activation energy for type A serrations, i.e. 79 and 119 kJ mol^?1 respectively. The results are discussed in terms of substitutional–interstitial solute atom interaction and changes of concentration of interstitial atoms.

MST/934     

Effect of temperature and strain rate on tensile flow and work hardening behaviour of a Ti modified austenitic stainless steel

Abstract

The tensile flow stress data for a 15Cr - 15Ni - 2.2Mo - Ti modified austenitic stainless steel in the temperature range 300 - 1023 K and in the strain rate range 6.3 × 10^-5- 1.3 × 10^-2 s^-1 was analysed in terms of the Ludwigson and Voce equations. It was found that the Ludwigson equation described the flow behaviour adequately up to the test temperature of 923 K, whereas the Voce equation could be employed over the full temperature range. The peaks/ plateaus observed in the variation of these parameters as a function of temperature and strain rate in the intermediate temperature range have been identified as one of the manifestations of dynamic strain aging (DSA). Also the variation of these parameters with temperature and strain rate could clearly bring out the different domains of DSA observed in this alloy. The work hardening analysis of the flow stress data revealed that, in the DSA regime, the onset of stage III hardening is athermal.     

Influence of deformation induced ferrite,grain boundary sliding,and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence of C on hot ductility in the temperature range 600–1000°C has been examined for three C contents (0·1, 0·4, and 0·75 wt-%). Using a strain rate of 3 × 10^?3 s^?1, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 × 10^?2 and 3 × 10^?4 s^?1 were examined. At the strain rate of 3 × 10^?3 s^?1, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the γ–α transformation. Thin films of the softer deformation induced ferrite formed around the γ grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae₃) and dynamic recrystallisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the 0·1%C steel corresponded to the presence of a large volume fraction of ferrite, this being the more ductile phase. For the 0·4%C steel decreasing the strain rate to 3 × 10^?4 s^?1 resulted in a very wide trough – extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding in the γ. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation induced ferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae₃, resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·1%C.

MST/1809     

Influence of test direction on hot ductility of austenite

Abstract

The influence of test direction on the hot ductility of a hot worked high S steel (0·15%S) has been examined over the temperature range 700–1100°C and for strain rates 3·3 × 10^?4 and 1·3 %times; 10^?2 s^?1. As with room temperature tensile testing, ductility was lowest in the short transverse direction and greatest in the longitudinal direction. Ductility troughs were observed for all directions at the lower strain rate. Increasing the strain rate improved hot ductility and removed the trough for samples tested in the longitudinal direction. Similar directional behaviour was observed for plate steel with standard S levels (≤0·01%), but the differences in hot ductility with direction were much reduced. Fracture was transgranular dimpled rupture when ductility was good, but intergranular for poor ductility. Intergranular failure occurred when the steels were austenitic. The elongated MnS inclusions were found to be situated in the γ-boundaries and it is believed that intergranular failure occurred by the inclusions restraining grain boundary movement and allowing voiding and decohesion at the inclusion/boundary interface, thus encouraging grain boundary sliding. Increasing the strain rate improved ductility by reducing the amount of grain boundary sliding and increasing the grain boundary migration rate making it difficult for cracks to link up.

MST/776     

Effect of strain rate on tensile behavior of carbon fiber reinforced aluminum laminates

In this paper, the tensile behavior of carbon fiber reinforced aluminum laminates (CRALL) has been determined at a strain rate range from 0.001 s^− 1 to 1200 s^− 1. Experimental results show that CRALL composite is a strain rate sensitive material, and the tensile strength and failure strain both increased with increasing strain rate. A linear strain hardening model has been combined with Weibull distribution function to establish a constitutive equation for CRALL at different strain rates. The analysis of the model shows that the Weibull scale parameter, σ₀, increased with increasing strain rate, but Weibull shape parameter, β, can be regarded as a constant.     

Computer modelling of phase diagrams

Abstract

An investigation has been made of the tensile behaviour between 20 and 600°C of two ultrahigh boron steels (Fe–2·2B and Fe–4·9B), consolidated by hot isostatic pressing at temperatures ranging from 700 to 1100°C. Tensile tests showed plastic deformation only in the Fe–2·2B alloy. A decrease in yield and ultimate tensile stresses occurred when the consolidation temperature was increased. This was accompanied by an increase in the elongation to failure. This alloy satisfies the Hall–Petch relation for all testing temperatures. The slope of the yield stress versus d^?1/2 curve (d is grain size) decreases as the temperature increases, indicating that the mechanism controlling plastic deformation becomes independent of grain size at high testing temperatures. The fracture mode observed was brittle at room temperature and ductile, shown by the presence of dimples, at temperatures above 400°C.

MST/2050     

Influence of B and Ti on hot ductility of high Al and high Al,Nb containing TWIP steels

Abstract

The addition of ～0·002%B and ～0·04%Ti as microalloying additions to improve the poor hot ductility and high risk of cracking on continuous casting of high Al containing twinning induced plasticity (TWIP) steels has been examined. Tensile specimens were either cast in situ or heated to 1250°C before cooling at 60 K min^?1 to test temperatures in the range 700–1100°C and strained to failure at 3×10^?3 s^?1. For tensile specimens reheated to 1250°C, the presence of B with sufficient Ti to combine with all the N improved ductility over the temperature range of 700–950°C, the reduction in area (RA) values being >40%. For the higher strength more complex high Al, TWIP steels having Nb present, there was no improvement in ductility with a similar B and Ti addition, when the average cooling rate after melting to the test temperature was 60 K min^?1. Reducing the cooling rate to 12 K min^?1 resulted in the RA values being close to the minimum required to avoid transverse cracking throughout the temperature range 800–1000°C. Using these additions of B and Ti, transverse cracking was found not to be a problem when continuously casting these high Al containing TWIP steels.     

Superplastic behaviour of as received superplastic forming grade IN718 superalloy

Abstract

Tensile specimens of superplastic forming grade IN718 superalloy, containing banded microstructure in the as received state, were deformed at high temperatures T to investigate the stress σ versus strain rate ? · behaviour, the nature of the stress versus strain ? curves, ductility, and microstructure upon failure. The log σ–log ? · plot for the ? · range ～5 × 10^-6–3 × 10^-2 s^-1 at T = 1173–1248 K exhibited a strain rate sensitivity index m = 0·62 at low strain rates and m = 0·26 at high strain rates, representing region II and III behaviour, respectively. The activation energies were estimated to be 308 and 353 kJ mol^-1, respectively. All the σ–? curves, obtained at ? · = 1 × 10^-4 s^-1 for the temperature range 1173–1273 K, and at T = 1198 K for the strain rate range 1 × 10^-4–1 × 10^-2 s^-1, exhibited initial flow hardening, followed by flow softening. The microstructures revealed dynamic recrystallisation, grain growth, cavitation, and a variation in the amount of second phase particles. Grain growth and cavitation were found to increase with temperature in region II. Excessive grain growth at 1273 K led to the elimination of region II. Grain growth and cavitation were both found to be less pronounced as the strain rate increased in region III.     

Effect of strain rate on tensile deformation and fracture behaviour of 18%Ni 300 maraging steel sheet

Abstract

Deformation and fracture under uniaxial tensile loading at room temperature were investigated for 18%Ni 300 maraging steel sheet in the strain rate range 1·67×10^?5 to 1·67× 10^?1s^?1. The steel showed an increase in flow stress with strain rate and the increase in yield strength (YS) was more pronounced compared with the tensile strength (TS), resulting in a corresponding decrease of TS/YS ratio. Both the level of deformation and the deformation zone were also reduced by the increasing strain rate. Fractographic analysis indicated that the increasing strain rate induced, to some extent, plane strain constraint in the sheet resulting in increasing fracture angle, decreasing ductility/fracture strain, and increasing dimple size. With increasing strain rate the work hardening rate dσ/d? and strain hardening coefficient (n value) of the steel also decreased; hence, correlations were found between dσ/d?, TS/YS ratio, and n value. The decrease of these three parameters caused strain localisation as confirmed by the presence of intergranular dimples and intergranular shear. Also, the dimple density decreased as the strain rate was increased.

MST/729     

Artificial neural network modelling to predict hot deformation behaviour of as HIPed FGH4169 superalloy

Abstract

The hot deformation behaviour of as HIPed FGH4169 superalloy was studied by single stroke compression test on MMS-200 test machine at the temperatures of 950–1050°C and the strain rates of 0·004–10 s^?1. Based on the experimental results, a back-propagation artificial neural network model and constitutive equation method were established to predict the flow stress of FGH4169 superalloy. The predictability of two different models was compared. The correlation coefficients of experimental and predicted flow stress with the trained BP ANN model and constitutive equation were 0·9995 and 0·9808 respectively. The average root mean square error (RMSE) values of the trained ANN model and constitutive equation are 0·39 and 2·21 MPa respectively. And the average absolute relative error (AARE) values of the trained ANN model and constitutive equation are 1·79 and 7·47% respectively. The results showed that the ANN model is an effective tool to predict the flow stress in comparison with constitutive equation.     

Tensile behaviour of type 304 austenitic stainless steels in hydrogen atmosphere at low temperatures

Abstract

The tensile behaviour of solution annealed type 304L, solution annealed type 304, and solution annealed and sensitised type 304 stainless steels was investigated in hydrogen and helium under a pressure of 1·1 MPa over the temperature range 300–80 K at strain rates ranging from 4·2×10^-5 to 4·2×10^-2 s^-1. For 304L steel, hydrogen environment embrittlement (HEE) increased with decreasing strain rate. For 304L and 304 steels, HEE increased with decreasing temperature, reached a maximum, and then decreased with further decrease in temperature: the decrease was particularly rapid near the minimum temperature for HEE. Sensitisation enhanced the HEE of 304 steel. Above the maximum HEE temperature, the HEE behaviour was similar to the hydrogen embrittlement behaviour of materials in previous studies, but near the minimum temperature for HEE it was different. Three types of hydrogen induced brittle fracture were observed as a result of HEE: transgranular fracture along strain induced martensite laths and twin boundary fracture on the fracture surfaces of solution annealed 304L and 304 steels, and grain boundary fracture on the sensitised 304 steel. It was found that from room temperature to the maximum HEE temperature, the HEE of the materials depended on the transformation of strain induced martensite and below the maximum HEE temperature it depended on the diffusion of hydrogen.     

Microstructure and mechanical properties of directionally solidified Al–Si eutectic alloys with and without antimony

Abstract

Hardness H, interjlake spacing λ, and tensile properties are reported for Al–12·7Si and Al–12·7Si–0·2Sb (all wt-%) eutectic alloys directionally solidified at growth velocities of up to 250 μm s^?1 and under temperature gradients in the liquid of up to 12·9 K mm^?1. The hardness is related to interflake spacing by the equation H=H^o+Kλ^?0·2, where H^o is the initial hardness of the alloy. This behaviour contradicts previous results, which suggest that a Hall–Petch relationship is followed. The tensile properties are shown to follow similar behaviour, confirming that hardness shows the same dependence as proof stress on interflake spacing. However, the nature of the relationship depends on the Si morphology and caution should be exercised in using hardness or interflake spacing to indicate proof stress.

MST/1585     

Effect of carbon content on plastic flow behaviour of plain carbon steels at elevated temperature

Abstract

In the present study the effect of carbon composition on the hot flow behaviour of two different plain carbon steels is analysed. For this purpose the constitutive equations describing the stress–strain (σ–?) relationships at a given strain rate ? and temperature T were determined for each steel. Uniaxial hot compression tests were performed to characterise the mechanical behaviour of the alloys. It was observed that irrespective of the test conditions, the low carbon steel displayed similar flow stresses to the high carbon steel. Comparison of the characteristic parameters of the constitutive equations describing the high temperature flow behaviour of these steels, together with values reported in the literature enabled determination of the effect of carbon content on flow behaviour. It has been found that flow stresses can be rationalised as a balance between work hardening and softening processes (basically dynamic recovery). At high temperatures and small strain rates, the high carbon steel showed lower hardening rates and slower dynamic recovery kinetics than the low carbon steel. In contrast, at low temperatures and large strain rates, the high carbon steel displayed higher hardening rates and recovery rates than the low carbon steel.     

Microstructure and tensile behaviour of 15–24 wt-%Mn TWIP steels

Abstract

The present study examines the microstructure and tensile behaviour of twinning induced plasticity steel with different Mn contents. The results show that reducing the Mn content from 24 to 15 wt-% is beneficial for the improvement of the tensile strength and fracture strain of the steels. The effect of Mn content on the microstructure and workhardening behaviour as well as the effect of strain rate on the microstructure and tensile behaviour is discussed.