Sept_AMP_Digital

ADVANCED MATERIALS & PROCESSES •

SEPTEMBER 2014

HTPRO

DEVELOPMENTS IN MICROALLOYED,

COARSENING-RESISTANT STEELS FOR

HIGH TEMPERATURE CARBURIZING

THERE IS GROWING INTEREST IN STEELS MICROALLOYED WITH TITANIUM, NIOBIUM,

AND MOLYBDENUM FOR USE IN HIGH-TEMPERATURE CARBURIZING APPLICATIONS.

Marvin McKimpson,*

Caterpillar Technical Center, Mossville, Ill.

There is growing interest in steels, par-

ticularly carburizing steels, that have im-

proved resistance to austenite grain

coarsening during heat treating. The

driver for much of this interest is

the increased use of low-pressure (i.e.,

vacuum) carburizing furnaces in com-

mercial heat treating operations. These-

furnaces are capable of processing steels

at higher temperatures and substantially

shorter cycle times than conventional at-

mosphere furnaces. The higher-temper-

ature capability is due to both furnace

construction and a lower potential of the

furnace to cause intergranular oxidation

in the steels being processed.

Figure 1 shows that increasing carburiz-

ing temperature from 950° to 1050°C

(1740° to 1920°F) potentially can shorten

carburizing time by up to 60%

[1]

. How-

ever, when carburized above 950°C,

many current commercial steels show

excessive austenite grain size coarsening.

For example, Fig. 2 shows the grain

growth in a commercially available mod-

ified SAE 4120 steel vacuum annealed at

1100°C (2010°F) for 10 hours. The large

grain size is apparent, with grains ap-

proaching 1 mm in size. Such grain

coarsening degrades both the toughness

and fatigue resistance of carburized

components, and is generally not accept-

able for commercial products.

Grain coarsening control

Most commercial carburizing steels rely

on submicron aluminum nitride (AlN)

precipitates to control grain size coars-

ening during heat treating. The nitrides

pin austenite grain boundaries, restrict-

ing grain growth. Unfortunately, at tem-

peratures above 950°C, the particles

begin to coarsen and dissolve, allowing

austenite grain growth. Research organ-

izations worldwide are exploring ways to

improve coarsening resistance of these

steels by incorporating submicron pre-

cipitates (generally either nitrides or car-

bides) having greater thermal stability

than AlN. The additional precipitates

both increase the volume fraction of par-

ticles available to retard austenite grain

boundary migration and resist precipi-

tate coarsening (i.e., Ostwald ripening)

more effectively than AlN.

Only a few elements, primarily titanium,

boron, and niobium, form nitrides, car-

bides, and carbonitrides likely to be use-

ful for improving austenite grain

coarsening resistance. Early research fo-

cused on using titaniumnitride (TiN) due

to its high thermodynamic stability. This

work showed that titanium additions to

steel can substantially improve grain

coarsening resistance. Unfortunately, as is

well known within the industry, even

small titanium additions often lead to

large cuboidal TiN or Ti(N,C) inclusions

that degrade both the toughness and fa-

tigue resistance of the steel. Accordingly,

using titanium for grain size control in

commercial alloys is likely to require very

careful control of steelmaking practices.

Recent work has focused on using nio-

bium, or a combination of niobium and

titanium, to improve austenite grain

coarsening resistance. Consider, for ex-

ample, a carburizing steel with microal-

loy additions of both titanium and

niobium. If titanium levels are suffi-

ciently low to avoid formation of large

cuboidal TiN particles in the melt (typi-

cally 0.02 wt% or lower), only small TiN

precipitate particles form in the steel as

it cools through the austenite tempera-

Fig. 1 —

Effect of carburizing temperature on carburizing time.

0.5 mm

Fig. 2 —

Grain growth in modified SAE

4120 steel vacuum annealed at 1100°C

(2010°F) for 10 hours. Source: Ref 2.

1050°C (1920°F)

1000°C (1830°F)

950°C (1740°F)

900°C (1650°F)

2 4 6 8 10 12 14

Carburizing time, h

3.0

2.5

2.0

1.5

1.0

0.5

Carburizing depth for 0.35%, CD

610

, mm

t ~ 60%

EHT

610

D = D

exp

(-Q/RT)

*Member of ASM Heat Treating Society