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Steam Treatment of Ferrous P/M parts

Steam treating, also known as steam oxidizing, is a low-temperature (510 ~ 570 C, 1 to 2 h

heat treatment pro!ess in whi!h "#$ steel parts are exposed to superheated steam to %orm a

la&er o% 'la!k iron oxide identi%ied as magnetite, in the sur%a!e porosit&, a!!ording to

the !hemi!al rea!tion)*e + 2. (steam / *e).+ 2(gas

he steam rea!ts with the iron sur%a!e !onerting it to an adherent, prote!tie 'luegre&

iron oxide (*e). 3e!ause the steam !an penetrate the porosit&, the oxide la&er !an 'e

0020 to 0050 in deep depending on the pro!essing !onditions

4team treating !annot trul& 'e des!ri'ed as a heat treatment 'e!ause no stru!tural

!hanges o!!ur in the matrix n the pro!ess, magnetite (*e). is %ormed at the

inter!onne!ting sur%a!e porosit&, %illing the porosit& with a se!ond phase $agnetite has a

hardness e6uialent to 50 C he pro!ess itsel% is straight%orward8 the primar& aria'les

are temperature, time, and steam pressure Caution must 'e exer!ised to preent the

%ormation o% h&droxides and lower oxide su!h as %errous oxide (*e. and %erri! oxide (*e 2.),

whi!h is red rust 9lso, adhesion o% the sur%a!e oxide la&er is an important aria'le to !ontrol

9dhesion is strongl& in%luen!ed '& pro!ess time at temperature

4palling or %laking o% the sur%a!e oxide la&er !an o!!ur i% the pro!ess temperature

ex!eeds 570 C and pro!ess times ex!eed h he maximum thi!kness o% the sur%a!e oxide

la&er should not ex!eed 7 :m 3e&ond this thi!kness, %laking !an o!!ur due to an in!rease in

sur%a!e tensile stress

he pro!ess !an 'e !ondu!ted in 'at!h, pressurized %urna!es, or !ontinuous 'elt

%urna!es

Steam treating enhances the product by:

; n!reasing wear resistan!e

; n!reasing sur%a!e hardness

; mproing !orrosion resistan!e

; n!reasing !ompressie &ield strength

; "roiding low-pressure leak tightness

*ormation o% this oxide la&er un%ortunatel& redu!es tensile strength and du!tilit& 10 to 20

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The recommended procedure for steam treating is:

1 "re!lean parts to remoe an& oil or lu'ri!ants that ma& hae 'een a'sor'ed into the

porosit& %rom prior ma!hining, sizing, or %inishing operations

2 =oad !lean sintered parts in loosel& pa!ked 'askets and pla!e %ixture into a %urna!e

preheated to )15 C) eat parts in air until the !enter o% the load has sta'ilized at the set temperature

ntrodu!e superheated steam at a line pressure o% )5 to 105 k"a (5 to 15 psi and allow

%urna!e to purge %or at least 15 min

5 n!rease %urna!e temperature to desired steam treatment temperature and hold %or no

longer than h at heat

> .n !ompletion o% the treatment, redu!e %urna!e temperature to )15 C (>00 * ?hen

parts rea!h this temperature, the steam !an 'e shut o%% and the parts unloaded

Caution should 'e used when opening the %urna!e door a%ter the steam !&!le 9s

indi!ated a'oe, h&drogen is produ!ed during this pro!ess and !an ignite t is re!ommended

that a nitrogen purge 'e applied prior to unloading his pro!ess, when !orre!tl& applied, !an

impart improed sur%a!e properties and, depending on steel !omposition, in!rease

!ompressie &ield strength

@6uipment must 'e properl& sealed to preent air !ontamination, or errati! oxide

%ormation, and dis!oloration !an o!!ur

n steam-treated "#$ steels, the du!tilit& is signi%i!antl& redu!ed due to the internal

stresses !reated '& the %ormation o% the iron oxide Care must 'e taken when steam treating

high-!ar'on "#$ steels 'e!ause these internal stresses !an initiate mi!ro!ra!king and !ause

seere loss o% du!tilit& he 'est re!ommendation %or preenting su!h an in!ident is to spe!i%&

a maximum !ar'on !ontent o% 05< %or "#$ parts that are to 'e steam treated

4intered densit& is an important !onsideration when appl&ing steam treating %or

improed strength and hardness ts a'ilit& to in!rease the wear resistan!e o% the su'strate

material depends on the aaila'le porosit& %or oxidation 9s densit& is in!reased, the amount

o% oxide %ormed is de!reased, whi!h minimizes the improement in apparent hardness

attri'uted to steam treating

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$i!rograph o% steam-treated stru!ture o% allo& *C-0205 (>75 g#!m) Anet!hed, 200B

e%eren!es

1 94$ olume 78 "owder $etal e!hnologies and 9ppli!ations