Small Punch Creep Test Data Research Articles

Heavy wall forged pipes of grade 92: effect of heat treatment conditions on microstructure and creep resistance

ABSTRACT The design of grade 92 steel seamless pipes has progressed significantly, but very little has been published on heavy wall forged pipes. In this work, microstructure and creep properties of 180 mm thick F92 pipes both normalised and tempered (N&T) and quenched and tempered (Q&T) are studied. Creep behaviour was assessed using both uniaxial tests and small punch creep test data analysis. Quenching treatment was able to develop cooling rates greater than 0.6 °C/s, limiting the presence of ferrite to less than 1%, while in case of normalising, the ferrite percentage was 3–4.5% (average cooling rate < 0.12 °C/s). Consequently, the Q&T steel exhibited creep strength of 20–25% higher than that of the N&T forged pipe of the same thickness. These results were exploited for the setup of a new proprietary heat treatment able to develop on heavy wall forged pipes’ creep strength levels similar to those of N&T thin seamless pipes.

Experimental and Numerical Analysis of Initial Plasticity in P91 Steel Small Punch Creep Samples

To date, the complex behaviour of small punch creep test (SPCT) specimens has not been completely understood, making the test hard to numerically model and the data difficult to interpret. This paper presents a novel numerical model able to generate results that match the experimental findings. For the first time, pre-strained uniaxial creep test data of a P91 steel at 600 ∘C have been implemented in a conveniently modified Liu and Murakami creep damage model in order to simulate the effects of the initial localised plasticity on the subsequent creep response of a small punch creep test specimen. Finite element (FE) results, in terms of creep displacement rate and time to failure, obtained by the modified Liu and Murakami model are in good agreement with experimental small punch creep test data. The rupture times obtained by the FE calculations which make use of the non-modified creep damage model are one order of magnitude shorter than those obtained by using the modified constitutive model. Although further investigation is needed, this novel approach has confirmed that the effects of initial localised plasticity, taking place in the early stages of small punch creep test, cannot be neglected. The new results, obtained by using the modified constitutive model, show a significant improvement with respect to those obtained by a ’state of the art’ creep damage constitutive model (the Liu and Murakami constitutive model) both in terms of minimum load-line displacement rate and time to rupture. The new modelling method will potentially lead to improved capability for SPCT data interpretation.

Small punch creep testing: review on modelling and data interpretation

In many situations where the characterisation of the mechanical behaviour of a specific material is required, source material for manufacture of conventional test specimens may be at a premium. Examples include the validation of new alloys for use in the power industry, the description of the heat affected zone (HAZ) of weldments or performing a remnant life study on an in service component (such as steam pipe work used extensively in the power generation industry). The potential for a limit in sample material has necessitated the development of small specimen designs and associated test methods, particularly for the determination of the creep behaviour of a sample material. The small punch creep test (SPCT) has the potential to characterise the full uniaxial creep curve (as the specimen is taken to fracture). It is for this reason that the small punch creep test has attracted much interest from the research community. Owing to the complex deformation mechanism interactions experienced in the small punch creep test, interpretation of the results has received attention from many authors since its application was proposed by Parker et al. in the 1990s (based on small punch plasticity test by Manahan et al. in the 1980s–). In this review paper, several methods for the interpretation of small punch creep test (SPCT) data are reported and compared, together with examples of their application. Considerations for finite element (FE) modelling of small punch creep tests are highlighted and critiqued. Recommendations for potential areas of future research are also presented based on the authors’ investigation into published literature and research.