Abstract
A 2205 DSS is a remarkable engineering alloy which has earned several industrial applications due to its superior mechanical properties. However, it’s tendency to hot crack during thermo-mechanical processing remain a major challenge. To alleviate this challenge, this research focused on two aspects associated with hot processing of 2205 DSS including: (1) investigating the hot flow behavior during single-pass deformation and (2) modelling of the phase balance as the function of imposed hot processing parameters. With regards to single-pass deformation, the aim was to understand the hot flow behaviour within the deformation zone, thus thirty (30) hot compression tests were performed at a temperature ranging between 850 - 1050°C. The first twenty-five (25) cylindrical samples were hot compressed to a total true strain of 0.8 using Gleeble 1500D that was programmed at various deformation rates of 0.001, 0.01, 0.1, 1 and 5s-1. The selection of the mentioned strain rates to conduct single pass deformation was in consistent with the fact that during hot rolling, strain rate variation exists between the entry and exit of the roll-gap. The remaining five (5) samples were also hot compressed to 0.8 true strain using Bahr 850DTM Dilatomer set at a higher deformation rate of 15s-1. On the other hand, the modelling of the phase balance was investigated by conducting a series of multi-pass hot compression tests. These tests were performed at both low (850°C) and high (1050°C) extreme temperatures, as well as low (1s-1) and high (15s-1) strain rates. Various hot rolling parameters were varied at these extreme deformation conditions in order investigate the evolution of the phase balance under various hot processing conditions.
The hot flow behaviour of 2205 DSS within the deformation zone was governed by restoration mechanisms that were in turn highly dependent on the deformation conditions. Low Z conditions ranging from [2.14 𝑥 1014 to 9.26 𝑥 1017]s-1 enhanced the restoration kinetics and brought softening by means of CDRX and DRX in ferrite and austenite respectively. Meanwhile high Z conditions above 4.54 𝑥 1018 s-1 also favoured the kinetics, but under these conditions the deformed austenite remained un-recrystallised while ferrite experienced DDRX. The investigated alloy also showed improved hot workability when the reduction per pass was kept between 16.5% to 42.3%. In addition to good hot workability, an optimum hot working window applicable to industrial hot mills was identified at a temperature ranging from [850 - 975] °C and the strain rates of [2.54 – 15] s-1. In this window, the hot processing of the alloy yielded no flow instabilities and mainly govern by DRV with no signs of DRX. Another hot processing window governed by full DRX was identified at low strain rates between [0.001-0.05] s-1 and temperature [925-1050] °C. However, the challenge with latter window was low
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strain rate range that was believed to be impractical for industrial hot rolling mills. Therefore, this hot working window was not ideal since the hot mill production rate may be negatively affected.
During multi-pass hot deformation, the 2205 DSS phase balance was found to be dependent on the imposed deformation conditions (strain rate, temperature, strain, interpass time). The increase in strain rate at both extreme temperatures led to overall reduction of austenite volume fraction. And based on the microstructural observations, this reduction was attributed to dynamic transformation of un-recrystallised deformed austenite into ferrite. Furthermore, longer interpass times at lower strain rate or shorter interpass time at higher strain rate respectively increased the ferrite volume fraction to 58% and 64%. Similar to interpass times, the strain influence on the phase balance also depended on the magnitude of strain rate applied. For instance, increased in strain at lower strain rates initially favour the formation of more ferrite at lower passes, however towards the last passes DT ferrite reverted back into austenite. The reversion process may have been initiated or favoured because of longer deformation times when the strain rates were lower. A different trend was observed at higher strain rates, where increase in strain reduced austenite phase fraction with no occurrence of retransformation. In this case, absence of DT ferrite retransformation was hindered by shorter deformation times at higher strain rates. Given the relationship between the phase balance and hot processing parameters, two empirical models catering for low and high strain rates hot processing were developed. In these models, the phase balance (austenite volume fraction) was modelled as the function of hot processing parameters. The findings revealed that high strain rate model which was more applicable for industrial hot rolling mills predicted as low as 36% phase fraction of austenite. This was only possible if the deformation conditions consisted of high strain rate and temperature, shorter interpass times, and progressive increase in strain per pass. From the industrial point of view, better hot workability and minimal risk of edge cracking in 2205 DSS are possible when the reversing hot mill is operated at a higher strain rate of around 15s-1 or more and interpass times of approximately 10s or less. This is due to presence of large volume fraction of softer phase (% ferrite ≈ 64) at these deformation conditions.