Predicting the mechanical properties and retained austenite content of ADI castings using non-destructive testing techniques

Abstract

Mechanical testing is expensive, labour intensive and only provides properties of sacrificial castings or representative test bars but offers no guarantees about other castings from the same batch. X-ray diffraction for determining retained austenite is slow in terms of sample preparation and testing and is not suitable for routine use. Hence the ability to predict mechanical properties and retained austenite contents of ADI castings by a rapid shop floor non destructive technique would improve in-house quality control and provide a means of evaluating every casting before despatch.

By combining multi-frequency eddy current data with ultrasonic velocity or resonant frequency measurements, clear relationships with 0.2% proof strength and tensile strength have been demonstrated. The best predictions of 0.2% proof and tensile strength for commercially produced ADI castings were better than±12% of measured values providing algorithms were derived using material having a similar composition and heat treatment. Under the same conditions the volume fraction of retained austenite in commercially produced ADI castings was predicted with an accuracy of better than ±0.04. An encircling eddy current coil is more sensitive than a probe and has slightly improved predictive accuracies.

Keywords:

• austempered ductile iron
• ADI
• mechanical properties
• multi-frequency eddy current testing
• NDT
• predictions
• retained austenite