A case study on failure and niobium-based enhancement of heater treater fire tubes made from SA387 Grade 22

Wear and corrosion of heater treater tubes at high temperature ∼ 500 °C in oil industries is a great problem of chromium/molybdenum low alloy steel (SA 387 Grade 22). These problems can be mitigated by alloying addition of specific rare earth metals in SA 387 Grade 22. Herein, we made alloying of 0.3 wt% and 0.5 wt% of Nb in SA 387 Grade 22 and investigated the effect on wear and corrosion properties. The sample A1 was unalloyed SA 387 Grade 22, while A2 and A3 contained increasing concentrations of Nb as alloying elements, specifically 0.3 % and 0.5 wt%, respectively. All samples were characterized according to the required characterization techniques, which can confirm the improved properties of A1. Scanning electron (SEM) microscopy revealed that the addition of 0.5 wt% Nb modified the microstructure of A3, which exhibited acicular ferrite and precipitates of niobium carbide (NbC). The A3 exhibited superior tensile strength of 879 ± 7 MPa (up from 580 ± 7 MPa in A1), Vicker hardness of 384.46 HV (up from 317 HV), and Charpy V-notch toughness of 344 J (up from 305 J), but ductility decreased from 24 % of A1 to 13 % of A3. Pin-on disc test demonstrated that A3 exhibited stable behaviour during the entire sliding time, resulting in a high value of coefficient of friction COF ∼ 0.656 (up from 0.591 for A1), directly indicating the improved mechanical strength, hardness due to 0.5 % Nb, with enhanced wear resistance. Barrier properties of A1 were improved by 0.5 % Nb addition in sample (A3), which shifted the corrosion potential towards a noble direction from (−0.507 ± 0.024 to −0.292 ± 0.014 V but also decreased the corrosion current density from −5.397 ± 0.025 to −6.105 ± 0.041 A/cm². A3, in which 0.5 % Nb addition was made, successfully improved the mechanical properties, wear and corrosion resistance of A1. These improved properties of A1 after 0.5 % Nb addition make A3 a suitable candidate for heater treater applications.