Lomna žilavost ledeburitnega kromovega jekla Fracture Toughness of Ledeburite Chromium Steel
S. Golubovič*, L. Kosec**
S poskusi smo ugotovili kritično vrednost faktorja intenzivnosti napetosti pri ravninski deformaciji za kromovi iedeburitni jekli Č.4150 in Č.4850. Jekli sta bili kaljeni in popuščeni pri temperaturah 180, 400 in 500 °C.
Kritično vrednost faktorja intenzivnosti napetosti K,c smo določali s polempirijsko metodo. Uporabili smo CT epruvete, v katerih smo z utrujanjem ustvarili začetne razpoke. S temperaturo popuščanja se lomna žilavost obeh jekel zmanjšuje. Jeklo Č.4850 ima skozi ves interval temperatur popuščanja boljšo lomno žilavost.
UDK: 620.178:669.15—196.58 ASM/SLA: Q6, Q7, Q26q, T5h, 2 — 64
The magnitude of the critical stress intensity factor in plane strain state vvas found out experimentally for ledeburite chromium steels Č.4150 and Č.4850. The two steel qualities vvere hardened and subsequentty tempered to temperatures 180, 400 and 500 "C. The critical stress intensity factor KIC vvas determined by a semi-empirical method. In the experiments CT-specimens vvere used vvhich vvere fatigued to create initial cracks. It vvas found out that vvith increasing temperature of tempering the fracture toughness of both steel qualities decreases.
1. UVOD
Visokoogljična in mnogolegirana orodna jekla imajo praviloma mnogo slabšo udarno in lomno žilavost od konstrukcijskih jekel. Ta jekla, vgrajena v orodja, morajo imeti visoko trdoto in z njo povezano obrabno obstojnost, visoko tlačno trdnost in mnoge tehnološke lastnosti, tako, da ostanejo v jeklu zelo majhne rezerve oz. prostostne stopnje, ki naj poskrbe za žilavost.
Pri raznovrstnih orodjih, ki se izdelujejo iz teh jekel, pa so tudi odpornost proti udarcem, koncentracijam napetosti in krhkemu, nenadnemu prelomu pričakovane lastnosti. Pri tej vrsti jekel ni tako velikih absolutnih povečanj obeh vrst žilavosti, kot jih dosežejo npr. konstrukcijska jekla na račun spremenjene kemične sestave ali toplotne obdelave. Lahko pa se na podoben način dosežejo precejšnja relativna povečanja, kar znajo uporabniki teh jekel ceniti. Udarna žilavost je podatek, ki že v veliko primerih dopolnjuje tradicionalno opremo diagramov popuščanja, o lomni žilavosti pa pri tej vrsti jekel ni kaj posebej izmerjenega. Vzrok so težave pri meritvah.
Ledeburitna kromova jekla so dobro znana in uporabljena za orodja, ki delajo v hladnem. Poleg klasičnih primerov poškodb zaradi obrabe se mnogo teh orodij tudi poruši.
Podatki o žilavosti pomagajo pri načrtovanju, izbiri jekel in njihovi toplotni obdelavi. Namen tega prispevka je pokazati rezultate poskusa izmeriti lomno žilavost gradiva, ki je po svoji naravi krhko in zavoljo tega predstavlja veliko težavo pri preizkušanju.
Lomno žilavost dveh značilnih predstavnikov kromo-vih ledeburitnih jekel, Č.4150 in Č.4850, smo merili pri temperaturi okolice po treh temperaturah popuščanja (180, 400 in 500 °C).
* institut za crnu metaiurgiju, Nikšič " FNT. Montanistika, Ljubljana
■ ■ Originalno publicirano' ZZB 22 (1988) 4 "" Rokopis prejet avgust 1988
1. INTRODUCTION
High carbon highalloyed tool steels are characteri-zed by a much lovver impact- and fracture toughness than structural steels. As tool components, these steels have to possess a high hardness and accompanying vvear resistance, a high compressive strength and many other technological properties, so that there are very fevv reserves left in the steel to provide it vvith toughness.
The variety of tools vvhich are manufactured of these steels requires the resistance to impact, stress concen-trations and sudden brittie fracture vvhich should also be counted among the expected properties. With this type of steel there are no absolute sharp increases in both ty-pes of toughness similar to those attained by structural steels due to their changed chemical composition or he-at treatment. It is, hovvever, possible by means of similar procedures to achieve considerable re lat i ve increases in toughness, vvhich is much appreciated by the users of these steels. impact toughness is an item of data vvhich has in many cases entered the traditional tempering diagrams vvhereas fracture toughness is not especially mentioned vvith this type of steel. The reason for this lies in experimental difficulties.
Ledeburite chromium steels are well knovvn and fre-quently used for cold vvork tools. Besides the classical types of damage due to vvear, many of these tools also experience fracture.
The data about toughness contribute to better plan-ning, and seiection of steels and their heat treatment. The aim of this paper is to present the results of the ex-periments and the measured values of fracture toughness of a material vvhich is characterized by nature as brittie and therefore difficult to test.
The fracture toughness of tvvo representative chromium ledeburite steel qualities Č.4150 and Č.4850 vvas measured at the ambient temperature after three different temperatures of tempering (180, 400 and 500 "Cj.
2. OPIS POSKUSOV
Preiskovani jekli sta imeli naslednjo kemično sestavo: (%)
C Si Mn	P S Cr Mo
4150 1,97 0,34 0,36	0,030 0,030 11,30 0,1 >CRI2)
4850 1,53 0,40 0,30	0,025 0,025 11,60 0,83
2. DESCRIPTION OF EXPERIMENTS
The investigated steel qualitles had the follovving chemicai composition:
(%)
C Si Mn	P S	Cr	Mo
Č.4150 1,97 0,34 0,36	0,030	0,030 11,30 0,1
Č.4850 1,53 0,40 0,30	0,025	0,025 11,60 0,83
Č, £
(OCRI2VM)
V Ni Cu Al Č.4150	1,53 0,19 0,15 0,016
(OCRI2)
Č.4850	1,18 0,17 0,22 0,049
(OCRI2VM)
Jekli smo stalili na zraku v indukcijski peči in ulili v ingote kvadratnega preseka 220 mm. S kovanjem smo dobili gredice kvadratnega preseka z robom 65 mm. Jeklo je bilo pred izdelavo epruvet mehko žarjeno.
Epruvete za mehanske preizkuse (trdnost, udarna in lomna žilavost) iz jekla Č.4150 smo kalili iz solne kopeli pri 960 °C v olje in jih dvakrat po eno uro popuščali na posameznih temperaturah; jeklo Č.4850 je bilo kaljeno na enak način s temperature 1010 "C in enako popušča-no. Mikrostrukturo litega, kovanega in toplotno obdelanega jekla smo preiskali z optičnim in transmisijskim elektronskim mikroskopom, količino primarnih karbidov smo izmerili s Ouantimetom 720, naravo sekundarnih karbidov z elektronsko difrakcijo, količino zaostalega avstenita pa rentgenografsko.
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Slika 1
Geometrija uporabljene CT epruvete s puščičasto zarezo Fig. 1
Geometry of the CT specimen vvith an arrovv-iike notch
V Ni Cu A/ Č.4150	1,53 0,19 0,15 0,016
Č.4850	1,18 0,17 0,22 0,049
The steels vvere melted in the air in the induction furnace and čast into ingots vvith square cross-sectional area (220 mm). Out of these, billets vvith square cross-sectional area vvere forged vvith the edge measuring 65 mm. Prior to the fabrication of the test specimens, the steel vvas annealed.
The specimens for mechanical testing (strength, impact and fracture toughness) made of steel quality Č.4150 vvere quenched from the salt bath at the temperature 960 "C into oii and tempered tvvice for one hour at each temperature. The steel quality Č.4850 ivas hardened in the same way from the temperature 1010 "C and also tempered in the same way. The microstructure of the čast, forged and heat treated steel vvas searched vvith the optical and TEM — the quantity of carbides vvas measured vvith Ouantimet 720, the nature of secondary carbides vvas studied by electron difraction, and the qu-antity of residual austenite from X-ray technique.
The magnitude of fracture toughness (stress intensi-ty factor) vvas measured by means of CT-specimens, the geometry of vvhich ensured a plane strain state. The specimens vvere cut out of billets so that the cut ran rec-tangularly to the direction of deformation and the tensile stress vvas acting in the direction of the deformation of the billet. The test bars vvere fabricated according to the ASTM E 399-83 standard, (1). The critical value of the stress intensity factor vvas determined semi-empirically by measuring the deformation on the CT-specimens (Figure 1) on vvhich primary cracks vvere initiated by fatigue on the MTS 820 machine. After the fracture it vvas examined vvhether the fatigue crack fulfills the con-ditions of the experiment.
After the static fracture, the length and the tip shape of fatigue crack vvere measured as well as the forces FQ and Fmax. From the force F0 ive calculated the assumed value of the factor KQ vvith the heip of the equation:
(2+ a/W)[0,886+ 4,64a/W- 13,32(a/Wf+ 14,72(a/Wf-5,6(a/VVf] \'(1-a/Wf
then the measuring conditions vvere controlled by calcu-lating the follovving parameters:
B,asr>2,5(-^f,	(2)
H pO. 2
vvhere B is the thickness of the specimen and a the length of the crack.
Betvveen the maximum value of the stress intensity factor Klmax in fatigue testing and the elasticity module, the follovving relationship has to hoid true:
0,00032 fm,	(3)
Velikost lomne žilavosti (faktorja intenzivnosti napetosti) smo merili s pomočjo CT epruvet, katerih geometrija je zagotavljala ravninsko deformacijsko stanje. Epruvete smo izrezali iz gredic tako, da je bila zareza pravokotna na smer deformacije, natezna napetost pa je bila v smeri deformacije gredice. Epruvete so bile izdelane po standardu ASTM E 399-83(1). Kritično vrednost faktorja intenzivnosti napetosti smo določali polempiri-čno z merjenjem deformacije na CT epruvetah (slika 1), na katerih je bila narejena primarna razpoka z utrujanjem na stroju MTS 820.
Po prelomu smo ugotavljali, če utrujenostna razpoka izpolnjuje pogoje poskusa.
Po statičnem prelomu smo izmerili dolžino in obliko čela utrujenostne razpoke ter izmerili sili FQ in Fmax. Iz sile FQ smo izračunali predpostavljeno vrednost faktorja KQ s pomočjo enačbe:
K0 =
B )A/V
Kfr
■ > 0,00032 Vm,
razmerje obremenitev: -p38* » 1,1
(3) (4) in
		Karbidi (%)		Zaostali avstenit (%)
Jeklo	Tpop	M23C6	M7C6	
Č.4150	180	10,5	89,5	10,2
	400	11,2	88,8	5,0
	500	11,2	88,2	0
Č.4850	180	8,8	91,40	10,9
	400	7,94	92,06	10,6
	500	7,91	92,09	7,2
Mehanske lastnosti obeh jekel so zbrane v tabeli 3:
the ratio of loads: > 1,1
Fq
(4) and
the size of the plastic zone at the tip of the crack vvhich has to be smaller than a 2% fatigue crack:
rpl < 0.02 as,
(5)
If the above four conditions are fulfilled, the assumed vaiue of the stress intensity factor Ka can be taken as the critical vaiue of this factor K,c.
(2 + a/W) [0,886 + 4,64 a/W -13,32(a/W); +14,72 (a/W)3 - 5,6 (a/W)4 j (/(1 -a/W)3
nakar smo kontrolirali pogoje merjenja še z računom naslednjih parametrov:
B, asr>2,5(^)2,	(2)
"pO,2
kjer sta B debelina vzorca, a pa dolžine razpoke. Med največjo vrednostjo faktorja intenzivnosti napetosti Kfmax pri utrujanju in modulom elastičnosti mora veljati odnos:
velikost plastične cone na vrhu razpoke, ki mora biti manjša od 2 % utrujenostne razpoke:
rPi < 0,02 asr	(5)
Če so izpolnjeni ti štirje pogoji, se privzame predpostavljena vrednost faktorja intenzivnosti napetosti KQ kot dejanska, kritična vrednost tega faktorja K|C-
3. REZULTATI
Mikrostrukturne sestavine obeh preiskanih jekel po toplotnih obdelavah so martenzit, primarni in sekundarni karbidi ter zaostali avstenit (slika 2). Količina in narava zadnjih dveh sestavin je navedena v tabeli 2.
Tabela 2:
Slika 2
Sekundarni karbidi v jeklu Č.4150 popuščenega na temperaturah a) 180, b) 400 in c) 500 "C Fig. 2
Secondary carbides in steel Č.4150 tempered at the temperatu-res a) 180, b) 400 and c) 500 'C
Tabela 3:
Jeklo	Tpop	Rm (MPa)	RpO.2 (MPa)	E(MPa)	Trdota HRC	Žilavost MJ/m2
Č.4150	180	1034	982	230000	62,8	0,078
	400	1024	973	230000	58,2	0,062
	500	1156	1098	230000	55	0,050
Č.4850	180	964	916	210000	62	0,075
	400	1033	981	210000	58	0,063
	500	1297	1232	210000	55	0,056
Tabela 4:
Jeklo	Tpop (°C)	•v (MPaj/m)	Kontrola
Č.4150	180	22,6	= K|C
	400	21,2	Kq = K,c
Č.4850	500	17,6	Kq = K,c
	180	33,3	Kq = K,c
	400	31,5	ka = K|C
	500	26,6	Kq = K,c
300 400 Temp. [°C1 Slika 3
Kritična vrednost faktorja intenzivnosti napetosti v odvisnosti od temperature popuščanja Fig. 3
Critical stress intensity factor K/c versus tempering temperature
4. Zaključek
Na način, ki je značilen za preizkušanje konstrukcijskih jekel, smo izmerili lomno žilavost dveh kromovih ledeburitnih orodnih jekel.
Osnovni problem pri preizkušanju je bil izdelati začetno razpoko z utrujanjem jekla. Izmerjeni faktorji intenzivnosti napetosti so odvisni od kemične sestave in mikrostrukture jekla.
Jeklo Č.4850 ima znatno boljšo lomno žilavost od jekla Č.4150. Pri obeh jeklih se lomna žilavost spreminja s temperaturo popuščanja jekla po kaljenju. Ta sprememba faktorja intenzivnosti napetosti je v dobri korelaciji s spremembo deleža zaostalega avstenita. Spremembe v
3. RESULTS
The microstructural components of both investiga-ted steels after the heat treatment procedures are: martensite, primary and secondary carbides and residual austenite (Fig. 2). The amount and the nature of the last two components are presented in Table 2.
Table 2:
Kritične velikosti faktorja intenzivnosti napetosti KIC pa so v tabeli 4. (Slika 3)
Carbides (%)
Residual austenite (%)
Steel	tPop	m23c6	m7c6	
Č.4150	180	10,5	89,5	10,2
	400	11,2	88,8	5,0
	500	11,2	88,2	0
Č.4850	180	8,8	91,40	10,9
	400	7,94	92,06	10,6
	500	7,91	92,09	7,2
The mechanical properties of the two steel qualities can be seen in Table 3:
Table 3:
Steel	Tp0p	Rm (MPa)	PpO.2 (MPa)	E(MPa)	Hardness HRC	Toughness MJ/rrf
Č.4150	180	1034	982	230000	62,8	0,078
	400	1024	973	230000	58,2	0,062
	500	1156	1098	230000	55	0,050
Č.4850	180	964	916	210000	62	0,075
	400	1033	981	210000	58	0,063
	500	1297	1232	210000	55	0,056
Finally, the critical values of the stress intensity factor K,c are presented in Table 4. (Fig. 3)
Table 4:
Steel	T tem (°C)	Ko_ (MPafm)	Control
Č.4150	180	22,6	KQ = K|C
	400	21,2	Kq = K|C
	500	17,6	= K,c
Č.4850	180	33,3	KQ = K|C
	400	31,5	KQ = K,c
	500	26,6	KQ = K|C
4. Conclusion
A method vvhich is typically used for testing structural steels vvas applied to measure the fracture toughness of tvvo chromium ledeburite tool steels. The basic problem of the testing vvas hovv to initiate a crack by fati-gue. The measured stress intensity factors are in depen-dence on the chemical composition and microstructure ofthe steel. Steel Č.4850 possesses a much higher fracture toughness than steel Č.4150. With both qualities of steel the fracture toughness varies vvith the temperature of tempering after the hardening procedure. This variati-on of the factor is in good correiation vvith the changing percentage of the residual austenite. The changes in the magnitude of the stress intensity factor are much more selective than the values of impact toughness measured
velikosti faktorja intenzivnosti napetosti so mnogo bolj selektivne od vrednosti udarne žilavosti, izmerjene po Charpyju na epruvetah z ostro V zarezo. Faktor intenzivnosti napetosti in vrednosti udarne žilavosti pri preiskanih jeklih se ne dajo povezati z znanimi empiričnimi obrazci.
Velikosti kritične vrednosti faktorja intenzivnosti napetosti za obe toplotno obdelani jekli dajejo v celotnem intervalu temperatur popuščanja prednost jeklu Č.4850. Te meritve posredno potrjujejo tudi znane vrednosti udarne žilavosti in prakso orodjarjev, ki dobro poznajo to prednost jekla Č.4850.
according to Charpy on test bars vvith a sharp V-notch. The stress intensity factor and the fracture toughness values of the investigated steels cannot be related to the knovvn empirical patterns.
The magnitudes of the ultimate values of the stress intensity factor for both qualities of the heat treated steel examined over the vvhole temperature interval of tem-pering give priority to steel Č.4850. Thus these measurements are also a indirect confirmation of the knovvn vaiue of the impact toughness and the practical experien-ce of tool makers who are well familiar vvith this advanta-geous feature of steel Č.4850.
LITERATURA/REFERENCES
1.	ASTM E 399 - 83, 519-542 (1983),
2.	Y. A. Geller, Tool Steels, Mir Publisher, Moscovv, 132—144 (1983),
3.	H. Berns, W. Trojahn. Einf. Der Warme behand. auf das er-mudung ledeb. kaltarbeits. 4 th Intern. Congr. on Heat Tre-atm. of Mater. Berlin, 427-439 (1985).
4.	J. Rodič, Mehanizem in morfologija lomov Cr-Mo-V orodnih jekel, disertacija, Ljubljana, 1978.
5.	E. Haberling, Hardenability of Ledeburitic Chromium Steels in a Vacuum Furnace, Thyssen Edelst. Techn. Ber. — 1983.