Fine Grained Boiler Tube


The objective of this work is to demonstrate a seamless tube fabrication method for obtaining uniform fine grained microstructures by a novel shear deformation process for tubular metal products. The manufacture of fine-grained RRR Nb superconducting radio frequency (SRF) cavities, and other tubular Nb products requires strict microstructure control with respect to grain size and texture for good formability. The major challenges in SRF cavity fabrication and performance stems from:

  • The high cost of pure Nb
  • A poor and inconsistent microstructure in the starting material
  • Seam welding to manufacture multi-cell cavities.

The approach presented by the authors indicates a possible strategy to obtain fine grain Nb tube by an innovative shear process. Grain size less than 30-m and tensile ductility greater than 40 percent in the orthogonal direction are achieved. The tensile properties correlate with the strongest texture component in the processed tube. Based on preliminary results, the proposed methodology maybe a viable and cost effective approach to fabricating a seamless Nb tube with good hydro formability.

Ultra-fine grained (UFG) cylindrical tube were produced via recently developed tubular channel angular pressing (TCAP) process through different passes from as-cast AZ91 magnesium alloy. The microstructure and mechanical properties of processed tube through one to four passes of TCAP process at 200 °C were investigated. Micro hardness of the processed tube was increased to 98.5 HV after one pass from an initial value of 67 HV. An increase in the number of passes from one to higher number of passes has no more effect on the micro hardness. Yield and ultimate strengths were increased by 4.3 and 1.4 times compared to those in as-cast condition. Notable increase in the strength was achieved after one pass of TCAP while higher number of passes has no more effect. Microstructural investigation shows notable decrease in the grain size to around 500 nm from the primary value of -150 -m. Dissolution and distribution of hard Mg17Al12 phase in the grain boundaries of dynamically recrystallized UFG AZ91 with a mean grain size of -500 nm was an interesting issue of TCAP processing at 200 °C compared to other severe plastic deformation processes.