experiment no. 7 done.doc

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EXPERIMENT NO. 7

HEAT TREATMENT OF STEEL

THEORY

The microstructure and resultant properties of steel are controlled by the composition

selected and the processing and heat treatment to which it is subjected. Heat treatment

generally consists of heating the cast or forged shape into the decomposition products:

1. Proeutectoid CAR!"# or $#RR!T#% depending on whether the carbon

content is more or less than the eutectoid composition for that alloy&

'. P#AR(!T#% which may be fine or coarse depending on the cooling rate&). A!*!T#% which may also +ary in scale& and

,. -ART#*!T#.

Executive Summary:-

!t is commonly belie+ed that high alloy castings ha+e inferior corrosion resistance to theirwrought counterparts as a result of the increased amount of micro segregation remaining in

the as/cast structure. Homogeni0ation and dissolution heat treatments are often utili0ed toreduce or eliminate the residual micro segregation and dissol+e the secondary phases.

"etailed electron probe microanalysis #P-A2 and light optical microscopy (3-2 were

utili0ed to correlate the amount of homogeni0ation and dissolution present after +ariousthermal treatments with calculated +alues and with the resultant corrosion resistance of the

alloys. The influence of heat treatment time and temperature on the homogeni0ation and

dissolution 4inetics were in+estigated using stainless steel alloys C*)-* and C5)-Cu*.

Autogenously welds were placed on the surface of the as/cast samples to determine thesignificance of the structural scale. 6olume fraction measurements and #P-A confirm that

enhanced homogeni0ation and dissolution 4inetics are achie+ed in the autogenously weldswhen compared to the cast structures due to the reduced dendrite arm spacing "A2. !nalloys% near/complete homogeni0ation and dissolution is achie+ed in the autogenously welds

at 1178 9C after only one hour due to the reduced "A. The cast materials on the other hand%

reuire a four hour heat treatment at 1'87 9C to achie+e comparable le+els ofhomogeni0ation and dissolution. $inally% it was demonstrated that the corrosion resistance of

alloys C*)-* and C5)-Cu* can be impro+ed to match the corrosion resistance of their

wrought counterparts. The influence of heat treatment time and temperature on the impacttoughness and corrosion resistance of cast stainless steel alloys C$/)% C$/)-% C$/;% and C$/

;- was also in+estigated. The impact toughness increased with increasing temperature and

time due to a combination of spherodi0ation and spherodi0ation followed by dissolution of

the continuous ferrite networ4 in the castings. Heat treatment was obser+ed to increasecorrosion resistance in the C$/)/H$ and C$/;/H$ alloys% but corrosion testing in the other

alloys did not result in a clear trend being formed with respect to the heat treatment.

Corrosion began at the interface of the ferrite and austenite phases and continuously progressed through the ferrite networ4.

O!E"TI#E

);


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"etermine the processing/microstructural relationship for the plain carbon steels 18,7 and


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magnesium. olution treatments .T.2 and age treatments were the two heat treatments

studied. #ach of these treatments was applied to a normal A)7= alloy and samples with

titanium and boron grain refining additions. The grain refined sample was harder aftersolution treatment. Dpon age both samples increased in hardness and both refined

and unrefined samples yielded nearly the same hardness. -icrostructure analysis showed

finer grains in the solution treated refined grain sample when compared to the unrefinedsample. After aging the microstructures were closer in grain si0e% but there were more fine

particles of silicon in the refined sample compared to larger particles in the unrefined sample

RES%LTS AN$ $IS"%SSION

Analy0e the photomicrography% Roc4well and Tu4on hardness measurements to identify the

microconstituents and phases present. Also relate the photomicrography and hardnessmeasurements to the heat treatment used. The attached TTT diagrams will be helpful in

e?plaining the results.

REFEREN"ES

1. amuels% (.#.% 3ptical -icroscopy of Carbon teels% -etals Par4% 3H: A-%1E;8% p. 7@; see also pp.)1=/)'E and ))'/))72.

'. As4eland% ". R.% The cience and #ngineering of -aterials% oston% -A:P #ngineering% 1E;,% Chapters 11 and 1).

). 6an 6lac4% (.H.% #lements of -aterials cience and #ngineering% 7th ed.%

Reading% -A: Addison/esley Publishing Company% pp. ,8'/,17% ,)E/,,7% ,77/,=E.

,. Callister% .".% -aterials cience and #ngineering% An !ntroduction% *ewFor4% *F: Gohn iley ons% 1E;7% pp. 1=7/1@;% 1E7/'1;.

7. Atlas of !sothermal Transformation "iagrams% Pittsburgh% PA: D% 1E71.

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HEAT TREATMENT S"HE$%LE

Austeniti0e all samples at ;=8°C 17;8°$2 for 1 hour.

Dse stainless steel foil to pre+ent o?idation during the heat treating.

()*+ample

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Temperature

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ample

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Temperature

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,=


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experiment no. 7 done.doc

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