METALLURGY & MATERIALS

Onsite replica of crack indication in cast iron dryer head

Early metallurgists studied the fracture surfaces of broken test coupons to determine if the cast structure of their cannons was proper. Today's microscopes and test machines allow for a more thorough study and evaluation of an alloy's structure and its performance in service. These same techniques, along with new techniques, are being used today to study non-metals. This field of science is very useful in evaluating many materials problems, both in analyzing failures and in predicting future performance.

Microstructural determinations are usually correlated with mechanical property tests; both published and specifically determined for the particular material. The results can be used to determine how a material will perform in future service, if it was properly made, if it has deteriorated in service, or if it has been over heated. Defects are frequently identified with a combination of metallography and SEM work.

X-ray dot map of sulfur concentration on an automotive leaf spring woody fracture surface which resulted in a loss of strength

Most castings contain imperfections. Metallurgical study can reveal the extent to which the casting imperfections actually reduce the service properties of the casting. Small casting imperfections may seriously degrade the properties of some castings and have no significant effect on other alloy castings.

Some examples of material investigations follow:

• The nature of crack like indications in cast iron dryer heads was investigated using on site replicas and cross sections through an actual head. Indications were determined to be trapped oxide films from the casting process.

• Questions on whether an electrical wire break was the result of a parting arc or high temperatures from a fire were answered by metallographic analysis of grain size changes.

Large porosity at the fracture surface in this bronze bushing was the result of high stress and not defects

• Metal fractures sometimes have a "woody" appearance. This leads to poor transverse fracture properties. It has lead to a premature spring failure and an unusual collapse of a welded structure.

• Analysis of porosity at and away from the fracture surface in a cast bronze valve stem bushing lead to the conclusion that the casting was properly made and failed under excessive loading.

Large range of grain size in wire provides evidence for arcing