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Metallographic Interpretation of Steel Forging Defects

Michael L. He

Scot Forge Company, Spring Grove, Illinois

Steel forgings are widely utilized in nearly every industry. They often play a critical role in theoverall design; therefore, there is a great need to understand the various manufacturing defects that

affect the steel forgings. Metallographic examination is a commonly used method of characterizing

forging defects. Most major steel forging defects have distinctive metallographic characteristics,thus making it possible to differentiate them metallographically. Certain mounting, polishing,

etching and viewing techniques are discussed relative to the type of defects presented. It should be

recognized that there are limitations of how far metallography can go in root cause analysis offorging defects, other characterization techniques such as SEM/EDS and microprobe analysis are

sometimes necessary to solve the problem.

The main steel forging defect types can be grouped as following:

Raw Material (Fig. 1): Exogenous inclusions are introduced into the steel during the pouring of thesteel ingots. Well block sand, runner brick, and teeming flux are the most common type found in

bottom-poured ingots. Indigenous inclusions (sulfide, oxides, etc.) are formed as a result of the steelalloying and/or residual elements; they are considered defects only when they fail their respective

acceptance criteria. Delta ferrite is a product of the steel solidification process that can be revealed

with several etching methods.

Forge (Fig. 2): Most forge surface defect contains forge oxidation and decarburization from

exposure to the forging environment. Lack of forge consolidation often appears porous (Swisscheese-like) and accompanied by noticeable dendritic segregation. Forge tears cannot always be

confirmed with metallographic examination alone but often can be suspected the metallographic

cross sectional view of the void has a torn appearance. Forge laps typically has an angle of 30-60

and is filled with forge oxidation and decarburization. Forge cracks and seams are likely oriented

nearly perpendicular to the forged surface. Hydrogen flake cannot be confirmed withmetallographic examination alone but has certain characteristics associated with it.

Heat Treat (Fig. 3): Quench cracks are a well know forging defect type. They occur during or

shortly after the quench and are characterized by intergranular cracks with the presence of temperscale and the absence of decarburization, assuming the heat treat furnaces used are oxidizing

atmosphere. Cooling rate from solution anneal temperatures can influence the formation certain

deleterious phases for duplex and super duplex stainless steel grades, e.g. Sigma phase. Severaletching methods have been developed to reveal Sigma phase.

Unusual Defect Types (Fig. 4): Several unusual defect types found in steel forgings are discussed.These include 4340 VAR ingot surface Mn segregation, post forge ductility crack, Al-Ti-Mn-Mg

oxides, Mn-Si oxides, Fe-Al inclusion, etc.

1050 CD

DOI: 10.1017/S143192760707105X Copyright 2007 Microscopy Society of AmericaMicrosc Microanal 13(Suppl 2), 2007

7/28/2019 Metallographic Interpretation of Steel Forging Defects

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Photomicrograph Examples of Steel Forging Defects

Fig. 1 Examples of exogenous inclusion such as teeming flux and well block sand.

Fig. 2 Examples of forge related defects: forge crack, lap, and forge non-consolidation.

Fig. 3 Quench cracks and Sigma phase.

Fig. 4 Unusual defects types encountered in steel forgings.

Teeming

flux

Well Block

Sand, Brightfield

Well Block

Sand, Darkfield

Forge Crack Forge Lap Forge Non-Consolidation

Quench Crack

From Surface

Quench Crack From

Center of a Bar

Mn segregation in

4340 VAR steel

Al-Ti-Mn-Mg

oxides Mn-Si oxides

Sigma Phase

Microsc Microanal 13(Suppl 2), 2007 1051 C