Elsevier

Materials & Design

Volume 32, Issue 2, February 2011, Pages 716-722
Materials & Design

Characterization of deformation instability in modified 9Cr–1Mo steel during thermo-mechanical processing

https://doi.org/10.1016/j.matdes.2010.07.038Get rights and content

Abstract

In this study, various existing instability criteria were employed to delineate the unstable flow regions in modified 9Cr–1Mo steel during hot deformation. Experimental stress–strain data obtained from isothermal hot compression tests, in a wide range of temperatures (1123–1373 K) and strain rates (10−3–10 s−1), were employed to develop instability maps. The domains of these instability maps were validated through detailed microstructural study. It has been observed that Hart’s stability criterion, Jonas’s criterion and Semiatin’s criterion under-predicts the instability regions in the studied temperatures and strain rates regime. Gegel’s and Alexander’s criteria as well as Murty’s metallurgical instability criterion, on the other hand, found to over-predict the instability domains. The instability map developed based on Dynamic Materials Model criterion has been found to precisely predict the instability domains. This instability map revealed four major unstable domains. Microscopic examination in these domains revealed that the instability is manifested in the specimens either as localized deformation band primarily along one of the diagonal or inhomogeneous distribution of martensite lath in the prior austenite grains.

Introduction

In Liquid Metal Fast Breeder reactor (LMFBR) power plant, the steam generator is considered as one of the critical components as the structural material is exposed to both liquid sodium and water. The failure of the structural material may lead to steam water reaction. In addition to this, the efficiency of the power plant largely depends on the thermal efficiency of the steam generator. Therefore, special attention needs to be paid towards selection of the material for steam generator in LMFBR. Modified 9Cr–1Mo steel which shows properties like good thermal conductivity, high temperature creep resistance, fracture toughness and corrosion resistance [1], [2], [3] has been chosen as the candidate material for the structural material of steam generator in Indian fast breeder reactors. This material is also being investigated worldwide for its potential application as in-core structural material in the next generation fast breeder reactors [4], [5], [6].

Once the material is selected, the performance of the component depends on the final microstructural condition which is controlled by the adopted thermo-mechanical processes [7], [8]. Hence attention has to be paid towards design of the thermo-mechanical processes by optimizing the process variables like temperature and strain rate. For this purpose, process modelling has been considered as a powerful tool. One of the prerequisites for the process modelling is the knowledge of the flow behaviour of the material [9]. Depending on the flow behaviour of materials, various regions in temperature and strain rate window could be broadly classified into two categories; stable and unstable domain. The regimes of temperature and strain rate where deformation is inhomogeneous and produce microstructural defects are termed as “unstable” or “unsafe” domains. The inhomogeneous deformation is unfavourable to the mechanical properties of the product, in particular ductility, fracture toughness [10]. In unstable domain, material may exhibit several kinds of instabilities depending on its processing conditions. At high temperature and low strain rates, material may exhibit superplastic deformation that may generate micro-porosity when it undergoes higher strain under tensile state-of-stress [11]. If deformation is carried out at low strain rate and high temperature regime, material may develop inter-crystalline or wedge cracks [12]. Given a favourable condition during service, this crack may propagate and cause catastrophic failure of the component. Similarly, when the material is deformed at higher strain rates, there is a possibility of local flow softening due to adiabatic temperature rise which usually causes localized slip. The intense flow localization may result in adiabatic shear bands. These bands are undesirable in the components as it may cause failure in the direction of shear bands. Therefore, it is important to distinguish the processing domains where the materials may exhibit unstable behaviour in order to avoid manufacturing of components with undesirable microstructure.

Plastic instability criterion has drawn considerable amount of interest after Hart’s investigation of stable flow in tensile deformation [13]. Following the Hart’s analysis of flow stability in tension, Jonas et al. [14] derived the instability criteria for the deformation under compressive loading. Semiatin and Lahoti [15] suggested a phenomenological criterion for predicting flow localization in hot forging of titanium and its alloys. On the basis of microstructural evolution, they have proposed a condition of α = 5 (α is the flow localization parameter) for the occurrence of flow localization in the material during hot deformation [16]. Kumar [17] and Prasad [18] have derived a criterion for instability known as Dynamic Material Model (DMM) criterion which has been successfully used for the optimization of process parameters of several materials [12], [19]. This criterion is based on the continuum principles as applied to large deformation proposed by Ziegler [20]. Using the foundations of continuum mechanics, thermodynamic and stability theory, Gegel and Alexander considered a Lyaponov function and suggested a set of criteria for the stable material flow [21], [22]. Murty and Rao [23] have also derived an instability condition based on the Ziegler’s continuum principles for delineating the regions of unstable metal flow during hot deformation. This criterion has been validated using the flow stress data of IN 718 with microstructural observations.

Since there is no unique instability criterion that can be applied to delineate the stable and unstable region for all kind of materials, designer needs to establish a suitable instability criterion or examine the applicability of the existing theories for the intended materials based on detailed microstructural observations [24]. The objective of the current study is to characterize the flow instability during thermo-mechanical processing of modified 9Cr–1Mo steel employing various instability criteria discussed in the preceding section viz. Hart’s stability criterion, Jonas’s criterion, Semiatin’s criterion, DMM criterion, Gegel’s and Alexander’s criteria and metallurgical instability criterion. The suitability of these instability theories on the studied steel would be examined with detailed microstructural investigation.

Section snippets

Experimental

Modified 9Cr–1Mo ferritic steel, used in the present study, was received in 1323 K/1 h normalised and 1033 K/3 h tempered condition. The chemical composition of the material is given in Table 1. Cylindrical compression test specimens of 10 mm diameter and 15 mm height were fabricated from the as-received plate. The hot compression tests were conducted using a computer-controlled servo-hydraulic testing machine (DARTEC, Stourbridge, UK) with a maximum load capacity of 100 kN. The machine is equipped

Flow behaviour

Flow stress of modified 9Cr–1Mo steel as a function of strain rate and temperature at 0.5 true strain is shown in Fig. 1. A progressive increase in the flow stress with strain rate is observed at all the studied temperature. This is an expected trend for strain rate sensitive materials and could be explained in terms of rate of dislocation generation/accumulation. The experimental flow curves of modified 9Cr–1Mo steel at three different strain rates (0.001, 0.1 and 10 s−1) are shown in Fig. 2.

Conclusions

Characterization of instability during thermo-mechanical processing of modified 9Cr–1Mo steel has been made employing various existing instability theory viz. Hart’s stability criterion, Jonas’s criterion, Semiatin’s criterion, DMM criterion, Gegel’s and Alexander’s criteria and Murty’s metallurgical instability criterion. Instability maps were developed based on these criteria to delineate the unstable flow regions employing stress–strain data obtained from isothermal hot compression tests in

Acknowledgement

The authors are thankful to Shri S. Sasidhara, Department of Materials Engineering, Indian Institute of Science, Bangalore, India for his help during compression testing.

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