August 2024 Volume 6

MATERIALS

surface cracking, unresolved casting defects, centerline burst, distor tion, and undesireable microstructure. This is followed by mapping the typical steps taken in evaluating a forging problem including: 1. Collection of background data and selection of samples for analysis. 2. Preliminary examination of failed part, typically visual exami nation, documentation of observations, photographic docu mentation (1x – 10x) 3. Nondestructive testing (NDT). 4. Selection, identification, preservation, and/or cleaning of critical specimens. 5. Macroscopic examination of fracture surfaces, secondary cracks and other phenomena (5x – 50x). 6. Analysis of external debris or oxides, if needed. 7. Microscopic examination of same by optical or electron micro scope (SEM) (25x – 10,000x). 8. Selection, preparation, examination, and analysis of metallo graphic sections by optical and electron microscopy. 9. Semi-quantitative analysis of chemistry in SEM or quantitative analysis in electron microprobe. 10. Comparison of microstructure to standards. 11. Hardness measurements and, if necessary, mechanical test spec imens from part. 12. Consultation with experts. 13. Formulate conclusions and recommendations.

Next, there is a quick tutorial of the Nondestructive Testing (NDT) techniques including visual inspection, ultrasonic testing (UT), magnetic particle testing (MP), and liquid penetrant testing (LP) which are often how a forging problem is first discovered. NDT Techniques and their application include: 1. X-ray inspection to look at internal passages, porosity, inclu sions and cracks. 2. Fluorescent penetrant inspection (FPI) to detect surface connected porosity or fine cracks. 3. Eddy current inspection (ECI) to look for surface and sub surface cracks. 4. Ultrasonic inspection (UT) to measure thickness and detect sub-surface defects. 5. Borescope inspection to examine the inside surfaces of internal passages. 6. CMM to measure dimensional changes as a result of deforma tion during service.

Forging Failure – Tooling Example:

Cause Grouping

Deficiencies Related to Failure

Die Material/Processing

Cleanliness, Forging Ratio, Mechanical Properties Corner Radii, Impression Shape, Die Working/Support Dimensions

This FIA Magazine Issue is dedicated to dies and die materials – so why a failure analysis article? The forging die may be one of the most important tools in the forging process; however, it is a consumable that needs be accounted for in production planning and die block pricing. The unplanned or prema ture failure of a die can result in produc tion interruptions, increased costs, decreased competitiveness in the market, and reduced economic benefits for the forging producer. The complex high temperature/high stress working conditions of forging dies lead to their failures also being complex. Failures wear and cracking of the cavity, thermal fatigue (thermal cracking), and plastic defor mation of the cavity surface or combinations of these. But, when a failure occurs, don’t immedi ately start the blame game with the die block

Die Design

Die Sinking/Machining Feature location vs. HT surface, Residual Stress, Finishing Die Use (and Re-Use) Ops Die Assembly/Loading, Die Preheating, Die Face, Work Material Heating, Die Lube

producers. Recent surveys have shown that the die or die block manufacture is rarely the cause of die failures in service. Although categorized as one of the main potential cause groupings (see Table), the die block material and processing is not typically found to be the root cause of a die failure in service. This is due in part to the continued improve ments in steel making (cleanliness), forging (grain refinement and flow), and heat treat ment (hardness and hardenability). Die designers capitalize on these improvements

by including the die block producer early in the die design and machining process. The failure analysis of the die must be done and is crucial in forming the best practices in processing by which the mean time between failures is increased. Die material still needs to be ruled out during a failure analysis, but more likely are the other potential cause groupings. The primary failure modes of wear and cracking of the cavity, thermal fatigue, and plastic deformation of the cavity surface are more likely to be traced to die design, machining and operation.

FIA MAGAZINE | AUGUST 2024 34