Creep Behavior of Sensitized AISI SS 347 Process Materials

Introduction

Hydro-processing units in refineries are used to convert heavier feedstock into more valuable light end products in a hydrogen-rich environment at very high pressures and moderately elevated temperatures [1]. Stabilized austenitic stainless steels such as AISI Type 321 and AISI Type 347 grade steels are typically used for fired heater tubes in hydrocracker units due to their excellent corrosion resistance, mechanical strength, and creep resistance [2]. The service life of these heater tubes may go beyond design life of 10 years or 100,000 hours depending on the material of construction, environment, and severity of operation at high temperature [3-4].

General austenitic stainless steels are susceptible to sensitization at high temperature operation when exposed to a temperature range of 600-800°C for a reasonable duration. During sensitization, formation of chromium (Cr) carbides at grain boundary regions and depletion of Cr at adjacent regions of grain boundaries occur [5-6]. To enhance their resistance to sensitization, chemically stabilized steels such as AISI Type 321 and AISI Type 347 are used. However, sanitization has been reported of these stabilized grades due to their long-term service and operational updates that result in overheating. In addition, sensitization in alloy AISI Type 347 at a temperature range of 500-600°C and time of more than 10,000 hours has been reported [7].

Robert J. Mills, et al investigated the effect of sensitization on creep response of 5083-H116 aluminum alloy through stress rupture studies. They found that sensitization does not have a significant effect on creep life of this material [8]. In this article, the influence of sensitization on creep response of hydrocracker feed fractionator fired heater tubes made of AISI Type 347 stabilized steel after service exposure of around 20 years is investigated.

Process Conditions

A creep life assessment of AISI Type 347 tube samples removed from a hydrocracker feed fractionator unit was evaluated after service life of around 20 years. The details of the operating and design parameters are given in Table 1.