Initiated by Air Products and carried out by TWI, this work aimed to confirm the integrity of bolts of different brass types assembled under varied conditions. These bolts form an intrinsic part of the plug on the valves of portable gas cylinder supplies and Air Products needed confirmation of materials composition, tolerances and assembly procedures that could be used without risk of failure. The investigation resulted in a recommendation on a particular brass alloy and assembly conditions to be used in valves in the company's internationally-used gas supply units.
The team from the Materials Department at TWI looked specifically at plugs that had been torqued to failure by Air Products for the purposes of the project. Analysis concentrated particularly on samples that had failed in low torque tests at around 10 Nm, compared with optimal designs that had failed at around 50Nm.
TWI viewed the fractured samples in detail using light and scanning electron microscopy. It obtained chemical analyses from those failing at both low and high torque, and studied samples of materials from the environment of the bolts using Fourier transform infrared (FTIR) spectroscopy. The team also inspected for residual stress in failed and unused bolt samples by using the mercury(I) nitrate test.
Investigation results showed that intergranular cracking occurred in those samples failing at a low torque, and that the crack morphology was characteristic of stress corrosion cracking. The environment in which stress corrosion cracks may occur will vary according to the type of brass and the chemical conditions experienced. Notably, cracking was not found in samples manufactured in 60-40 leaded brass and where the chemical environment was controlled to eliminate potential sources of stress corrosion.
Successful completion of the test programme meant that Air Products has been able to specify assembly procedures and conditions, chemical treatment and environment when using 60-40 leaded brass in valve units for its portable gas supplies: protecting against costly failure and leakage problems.
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