February_AMP

ecarburization is detrimental to

the wear life and fatigue life of

steel heat-treated components.

This article explores some factors that

cause decarburization while concentrat-

ing on its measurement. In most produc-

tion tests, light microscopes are used to

scan the surface of a polished and etched

cross-section to find what appears to be

the greatest depth of total carbon loss

(

free-ferrite depth,

or FFD) and the great-

est depth of combined FFD and partial

loss of carbon to determine the

maxi-

mumaffected depth

(MAD).

In some cases, there is no free

ferrite at the surface. In research stud-

ies, this may be supplemented with a

Knoop hardness traverse to determine

the depth where hardness becomes con-

stant. The Knoop-determined MAD is of-

ten somewhat deeper than the visually

determined MAD, as variations in the mi-

crostructure of carbon contents close to

the core may be difficult to discern. The

MAD determined by hardness traverse

may be slightly shallower than that de-

termined by quantitative carbon analysis

with the electron microprobe. This is es-

pecially true when the bulk carbon con-

tent exceeds about 0.45 wt%, as the rela-

tionship between carbon in the austenite

before quenching to formmartensite and

the as-quenched hardness loses its linear

nature above this carbon level.

Decarburizationbasics

Decarburization occurs when car-

bon atoms at the steel surface interact

with the furnace atmosphere and are

removed from the steel as a gaseous

phase

[1-8]

. Carbon from the interior diffus-

es towards the surface, moving fromhigh

to low concentration and continues until

the maximum depth of decarburization

is established. Because the carbon dif-

fusion rate increases with temperature

when the structure is fully austenitic,

MAD also increases as temperature rises

above the Ac

. For temperatures in the

two-phase region, between the Ac

and

, the process is more complex. Carbon

diffusion rates in ferrite and austenite

are different, and are influenced by both

temperature and composition.

Decarburization is a serious prob-

lembecause surface properties are infe-

rior to core properties, resulting in poor

wear resistance and low fatigue life. To

understand the extent of the problem,

two characteristics that may be present

at a decarburized steel’s surface can be

measured: Free-ferrite layer depth (FFD,

when present) and partial decarburiza-

tion depth (PDD, when free-ferrite is

UNDERSTANDING

ANDMEASURING

DECARBURIZATION

Understanding the forces behind decarburization is the first step toward

minimizing its detrimental effects.

George F. Vander Voort, FASM*, Struers Inc. (Consultant), Cleveland

Life Member of ASM International

Fig. 1

—Decarburized surface of as-rolled, eutectoid carbon steel (Fe-0.8%C-0.21%Mn-0.22%

Si) at two different locations around the periphery show a substantial variation in the amount

and depth of ferrite at the surface. The matrix should be nearly all pearlite (4% picral etch, 500×).

Fig. 2

— Erratic depth of free ferrite at

the surface of a bar of 440A martensitic

stainless steel after quenching from 2000°F

(1093°C); etched with Vilella’s reagent.

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