SCREWS AND FASTENERS

Problem Solving

Screws/Bolts/Nuts/Nails

A number of failed screws were examined for a manufacturer of steel grating to determine the reasons for failure. SEM, metallographic and Rockwell hardness measurements were used to determine that the screws failed due to hydrogen embrittlement.

The following examples illustrate some of our contributions to this business sector:

  • Touchstone was asked to determine the cause of a bolt failure. Microstructural evaluation found that the failure was the result of laps which can develop during roll-threading during manufacture of the bolts. The laps present a stress concentration point which leads to the failure of the bolt during use.
  • Corrosion resistance testing on a rear handle rivet, handle washer, pivot bolt, pivot nut, and thrust plate was performed to qualify materials for military requirements. The test was standard B117 salt fog exposure for 100 hours.
  • Three grade B-7, dryer bolts were evaluated to determine the cause of failure of a broken bolt. The cause was found to be tool marks and/or cracks left during machining of the threads which propagated to failure under operating stresses. The use of grade 8 bolts was recommended, with threads rolled after heat treatment, to prevent problem.
  • Failure Analysis of Cadmium-Plated Screws. A number of failed screws were examined for a manufacturer of steel grating to determine the reasons for failure. These screws were used to hold down the aluminum nosing on steel-grating stairs. The screws were failing at a number of locations throughout the country. All of the screws that were failing were from the same original lot. SEM, metallographic and Rockwell hardness measurements were used to determine that the screws failed due to hydrogen embrittlement. The most likely source of the hydrogen was the electrolytic cadmium plating process.
  • Five mine roof plates that failed in service, and four plate washers, were examined as part of a mine failure investigation. Touchstone engineers concluded that cracking at the bolt hole resulted from circumferential tensile stresses that developed from distortion of the plates after installation.
  • A failure of a compressor was traced to the use of substandard bolts used to hold the main bearing cap in place. The bolts were grade 8 fasteners, but the threads had been rolled before heat treatment instead of after. While there does not appear to be a requirement to roll thread after quench and tempering, it has been clearly shown in the literature that the greatest benefit will be achieved by threading after heat treatment. This adds residual stress to the threads and imparts greater fatigue life.
  • A cut nail Plant reported excessive knife wear on nail cutting machines used in their nail manufacturing process. TRL gave a thorough metallographic examination of the samples and determined that a significant amount of mill scale was found on and in the concast material surface after shotblast cleaning as compared to material before shotblast cleaning. Additionally, the shot blast operation work hardened the surface. The surface condition resulted in excessive knife wear. It was recommended that steps be taken to increase the efficiency in the scale removal process. One alternative was the use of abrasive grit instead of steel shot.
  • Touchstone was contacted concerning the evaluation of several grades of stainless steel. The results were to be used as a guide in material selection for the manufacture of stainless cut nails for outdoor applications. It was concluded that, based on the reaction of the various nails to the salt fog exposure, a nail manufactured form a particular stainless steel alloy was found to be least reactive. It exhibited little staining on the exposed surface and the least amount of discoloration on the contact surfaces and on the underside of the wood panels. A comparison of the holding power of these stainless steel nails to standard wire nails was also performed.
  • Touchstone was requested to evaluate lighting fixture specimens in order to determine the cause of corrosion and to suggest changes in manufacture which would assure the satisfactory performance of new units. TRL concluded that the rusting on the light fixtures was due an uneven surface on the steel base, slivers on the steel surface, and also very thin coating which did not provide adequate protection of the base metal. The rusting on the fixture screws was attributed to the same conditions as that of the fixture.
  • Eleven metal samples were sent to TRL for evaluation. These included a pole piece and a gap piece from a speaker, 3 wood screws, 3 spacer bars and 3 pieces suspected of being Alnico magnets. The speaker parts were examined to determine what type of metal was used in their manufacture, how they were manufactured, their density, and what type of plating might be present. The remaining samples were evaluated to determine what type of metal was used in their manufacture. Additionally, the specimens were examined to determine whether any surface treatment such as chromate conversion coating was present. All pieces were found to be 1010 steel, some were zinc electroplated, and chromate conversion coated.
  • The fracture surfaces of failed screws were examined to gain information on the mode of failure. SEM micrographs were taken of the fractured surfaces. Analysis of the micrographs of the failed screws revealed that there was a mixed mode of failure (brittle and ductile). The conclusion was that hydrogen embrittlement from plating was the most probable cause of the failure.
  • Expert testimony for legal action over the failure on installation of steel fasteners. The project included courtroom testimony. The manufacturer of the screw claimed the failures were the result of stress-corrosion cracking caused by over torqueing of the screws upon installation. Our analysis suggested hydrogen embrittlement – a manufacturing flaw. The client, based on our testimony, was awarded an amount equal to the replacement cost of the screws, plus legal and engineering fees.

The above examples are only a sample of our work in this area. We would be pleased to provide any additional information you may require. We also invite you to review examples on our website of work that we have performed in support of other business sectors.