4.3.1 Introductlon:
Microhctrdncss measurementS both on the surface and ilS il func- tlon of tilstance from the surface hilVe been n,ade on all samples.
Hardness versus pelletratlon-dlstallCe pro£ll06 glve useful lnformation regarding the depth of penetr<ltion of llltrogen
<lnd the v"riation of hardness from the surface to tne core <lnd iire used extenslvely. The results of hardness measuren.ents
obtillned in these studies ilre discussed below.
4.3.2 Surface hardness
Surface hardness measurements "-ere c<lrried out on all llquld- nltrlclect san'ples by using;;! Simuuz>.:mlcroharclnes" tester.
'l£)
~OgrhJ.~d~~ l1scd fer 5 s,=c to cause the ident"ticns •
.I:'lg. 4.6 shows that the 8urfaee hardness of il11 nltricieci
8amples falls grildually wlth treatm0nt time at all teml,erill:.ures lnvcstigated. Horeovur, it hus been observed that tbe hardness values (VHN) obtained On sanlples treated dt lOl';8r l:emper;;!ture",
(5250CI ilre alway", higher tnan the surf Lice hardness obtalned on samples trei1ted ilt h,igner temperatures (550°C and 57SoCI.
'1''18rate of decre;;!se in surface h,>rdnes5 lS greater at nigher tCIc.per"tures.
Hardness ir. nitriding is causee. by finely ulspersed nitride particles .wlthln the lr.atrlx and l5 dependent on the size of
"
600
o - 525°C
1
t;; - :.500C
0- 575°C 700
~ uw
~
~
600
~ 0~ 0 40
~ E
500
~
0
~
-
0
z 400
~
~ w
z
0
~
300
4~
~ W~ u
> ZOO
- (100,0) 0.5 1.0 2.0
TREATMENT
3.0 _ 4.0
TIME HR ---
6.0
FIG.4.6 VARIATION IN SURFACE HARDNESS WITH TREATMENT TIME OF MILD STEEL SPECIMENS LIQUID NITRIDED IN UREA- SODA BATHS.
•
eh" nitl:"ide particles. Pill lIpS and seybolt17 stuci"d pr"cipi- tatlon of alloY-Illtrides wnng electroll IlIIcroscol'Y and note,i tllilt coar"e nltrides caused only small to moderate haruefllIlC;.
Thuy identIiIud tiny particles, ,",'t11ch """ere nard to resolve, to be th", principle source of harden1n",.
'ille gl:owth of the nitride particles is depQnded on time and temperature and therefore th" rate of growth is likely to be higher at higher temperatures, thus. c.ausing. a faster Cleerease
i,n hardness at higher temperatures.
4.3.2.1 Eff8Ct of sulfur on surface hardness:
Pig. 4.7 show:; that the addition of sulphul: in l1qU1d nitrl- ding baths causes an increase in surface h"rdness as con,pared to the surface hardness of the mIld st8cl sampl,,'s nitridec
at the san,e temperature in oaths without sulphur. It ~" 5een therefore, that additions of sulpnur increases thc surfacu hardness.
4.3.3 Hardncss versus pcnetrat~on distance profiles
Seetloll orthogonal to tlw treatod surfac<2s were nou)lted and wel:e used for rni,croilardness me"surewents as a funetloll ef
dL"tam;e from the edge. The zero l'0lnt for the dist"nce sca'le corresponds to the extreme outer edge of the san'l'l" including
the compound layer. The llardness measurements st"rted at tJ',e point of closest approach to the edge and Wel:e spacea equally
:;,
,
700
I 0-
(2575°C
Pet. Sulphuraddl?d bath)
600 .- 575°C
uw
~
~
~
0
500
~ 0
~ E~
400
00
- -
0
z
~
300
~w
z
0~~
x
~
200
w<
~
>
(100,0) 0.5 1.0 2,0
TREATMENT
3.0
TIME (hr.)
40 - 5.0 6.0
FIG.4.7 EFFECT OF SULPHUR ADDITION ON SURFACE HARDNESS OF STEELS LIQUID NITRIDED IN UREA-SODA BATHS,
700
o -
G.5h• - 1.0 h
6.-2.0h• - 3.0h
El - 5,Oh
a g 400
E~ oo
-- .300
"
~~w
z
~ 200
~
I
~w
is
>100
{O,G}
0.2 04 0.6 08 10
DISTANCE BELOW SURFACE {mm) •
FIG,l,.a VARIATION IN HARDNESS VERSUS PENETRATiON DISTANCE
PROFILES WITH TREATMENT TIME OF MILD STEEL"
SPECIMENS LIQUID NITR1DED AT 5250 C IN UREA~SODA
q)A:HS.
700
I 600 6_ o -• -, - ".Oh
C.Sh2 ,Oh 3.0h
~
0-
5.0h
uw
~
~
oc 500
0~ 04 0~
~
E
0
'"
~ 0
z
~~ W Z0 OC
4I
200
OCw u~
>
100
0,0 0.2 0.8
DISTANCE
0.4
BELOW
0.6
SURFACE (mm) "
10
FIG.li.9 VARIAfION IN HARDNESS VERSUS PENETRATION 'DISTANCE PROFilES WITH TREATMENT TIME OF MILD STEEL SPECIMENS LIQUID NITRIDED AT 550"C IN UREA-SODA BATHS.
~
;
700
0- O.5h 0- 1.0h
C>- 2.0h
0-
3.0h 600
0- 5:0h
-
uw
~sao
~
~0
~ 0
~
, 00
0~ E~
0
0
- 300
0
z
~~
w 200
z
0~
~
I
~
W
~
100
u
>
0,0
0,2a' o
60,' 1.0
DISTANCE BELOW SURFACE (mm) .
F1G.4.10 VARIATION IN HARDNESS VERSUS PENETRATION DISTANCE PROFILES WITH TREATMENT TIME OF MILD STEEL SPECIMENS LIQUID NITRIDED AT 575DC IN UREA-SODA BATHS.
Q;
",
700
• '- 525"C
G - 55QoC
I
8-.575°C 600'
~ uw
~
soO
~
~ 0~
Q
~
400
9
E
~ 0 0-
0
300 z
~
~w Z
Q
ZOO
~
~
I
~ W~ u
100
>
(0,0) 0.2 0.4 06 0.8 1.0
DISTANCE BELOW SURFACE (mm) ~
FIG. 4.11 VARIATION IN HARDNESS VERSUS PENETRATION DISTANCE PROFILES WITH TREATMENT TEMPERATURES OF MILD STEEL SPECIMENS LlQUtD NITRIOED FOR 2 HOURS IN UREA-SODA
\;jA.THS.
25
as far as possible. Fig. 4.8 to Fig. 4.11 sno'", the varlatlon in hardness versus penetration distance profiles witn treat- [[ent time at constant temperatures and wIth teIr.perature for a fixed treatment tinle for mild s,teel samples llltrided in urea-soda baths.
From these figures it can be observed that case depth, i.e.
the distance at WhIChthe slope of the hardness versus penetra- tion distance profiles becomes minimum, increases Wit~l inc- reasing treatment tin\e at a par'ticulilr temperature and also wi.th temperilture ilt constant tn"atment time. Tnis is tQ De ex;)ucted. It can also be noted that wlth increasing treatment
time at a particular temperature the hardness curves nas a more gradual slope from surface to the core. This is DecaU"e of the uecre<l-sein concentr<l-tion gradient of nitrogen '.-lit"
increilsing nitriding time. A comparison9 of case-depths produced by nitriding in conventional baths and in oaths based On UrCa- soda, hilS shawn that nitriding in urea-soda baths produces
il greater depth. The deeper penetratlon of nitrogen durlllg nitriding in urea-soda baths resulting in tne hlgher casu depths h~s been attributed to the formation of an~onia gas ,ill the b~th.
Fig. 4.12 shows the variation in Cilse depth with treatnlent time fur tne mild stoel SIX'ClTILCns.It eeln De Qbsetved ~IJat ease- depth increilses with i,llcreilSi,Jlgtime at constant tenLperaturcs and ~l~o with lncreasing tcmpcratllrus at constant tiwe. Tile
e- 575°C [J-550"C
8- 525°C
1.0
08
E 0.6
~
E
I~
~ W
0
0.4
w "
~u
~
0.'
(0,0) 0.5 1.0 2.0 3.0 4.0 5.0 6.0
TREATMENT TIME (hr.) •••
FIG.4.12 VARIATION IN CASE DEPTH WITH TREATMENT TIME AT DIFFERENT TEMPERATURE OF MILD STEEL SPECIMENS
LIQUID NITRIDED IN, UREA- SODA BATHS.
700
600
-
uw
500
~
,
~
a
~
a '00
~ 0~ E~
a a laO
-
~ 0
z
~~ w
z '00
0
~
~
I
~ W
,
'00
u
>
G - O,Sh
• _ loOh
6- 2.0h Q1- 3.0h.- S.Oh
(0,0) 0.2
DISTANCE
O. ,
BElJ)W
0.6
SU~FACE (min)
0.6
•
'a
FIG.4.13 VARIATION IN HARDNESS VERSUS
P.ENE.TRATION"DIS:TANCE
PROFILES"WITH TREATMENT TIME OF MILO_STEEL LIQUID(~NITRIDED AT'S75°C IN" URE~- SODA BATH WITH THE
'llADDITION OF 2 Pet, SULPHUR,