Metal Science and Heat Treatment Vol. 38, Nos. 9 - 10, 1996

HEAT TREATMENT WITH HIGH-ENERGY SOURCES

UDC 621.9.044:620.186:669.14

EFFECT OF PLASMA-ARC TREATMENT ON STRUCTURAL TRANSFORMATIONS AND SURFACE HARDENING OF CARBON AND ALLOYED STEELS

D. S. Stavrev, t L. M. Kaputkina, ~ S. K. Kirov, ~ Yu. V. Shamonin, t and V. G. Prokoshkina ~

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 16- 19, September, 1996.

Plasma-arc surface treatment is a rather simple and effective method for hardening steels. The increase in the resistance to external effects depends on the treatment parameters, the composition of the treated alloy, its initial structure, and the preliminary and post-heat treatment. The present work is devoted to the effect of the regime of plasma-are surface treatment of steels with different initial structures on the proportion and extent of the strengthened layers, the distribution of the phase and structural components in them, and the wear resistance.

We studied specimens 30 x 30 x 10 mm in size of steels 45, 40Kh, U8, KhVG (Table 1) subjected to a preliminary heat treatment, namely, toughening. Surface treatment was performed by two regimes on an installation that provided hardening by a scanning plasma arc at a frequency of 20 Hz, namely, regime 1 with a current strength I= 140 A and a ve- locity of plasmatron displacement vp = 420 mm/min and re- gime 2 with I= 120 A and vp = 220 mm/min. The hardened zone was 20 mm wide and 0.7 - 1.3 mm thick.

A comparative volume hardening was conducted for

specimens 15 x 15 x 10 mm in size atter heating in an indif- ferent medium to 850C and a hold for 25 min. Then the specimens were ground and polished by mechanical and elec- trolytic methods (the composition of the electrolyte was 195 ml H3PO 4 + 33 g Cr203 + 7 mi H20) under conditions pre- cluding heating above room temperature.

Moscow Institute o f Steel and Al loys, Moscow, Russia.

X-ray lines of martensite and retained austenite were ob- tained on a DRON-4S-01 diffractometer in cobalt Ka radia- tion at points of layers lying 50, 400, and 700 lam from the surface. The profile of the martensite line (200) was separated analytically into singlets using the method of Shtremei' and Kaputkina [ 1 ]. The lattice parameters c and a and the amount of tetragonal tempered (self-tempered) martensite were deter- mined. The lattice parameters of austenite were determined from the centers of gravity of the lines (h/d) and % by linear extrapolation of the Nelson-Ri ley function to an angle of 90 . The random error in measuring the lattice parameter for each line (h/d) was estimated for a confidence level of 0.95 with allowance for: (1) the error in measuring the back- ground, (2) the error determined by the volume of reflection statistics, (3) the error due to the chosen methods for record- ing the profile of the line and for pointwise calculation of the coordinates of its center of gravity.

Foils were prepared for an electron-microscopic investi- gation in the following way. Plates 0.8 mm thick were cut on

TABLE I

Concentration of elements, % Steel

C Mn Ni W Cr Si S P Cu

U8 0.76 - 0.83 0. I 7 - 0.33 0.25 - < 0.20 0.17 - 0.33 0.028 0.030 0.25

45 0 .42-0 .50 0 .50-0 .80 0.25 -

Effect of Plasma-Arc Treatment on Structural Transformations of Carbon and Alloyed Steels 383

a Microslice installation and then ground to a thickness of 100 !am. Then they were subjected to electrolytic polishing in "closed" pincers in an electrolyte at 18C under a constant voltage of 30 V. The structure was investigated under a Neo- phot-33 microscope and a electron Tesla BS-540 microscope.

The wear resistance was determined from the loss of mass in the specimens under a contact load of 3.5 N/mm 2 by the body-counterbody method. The loading cyclicity was 120 cycles/min.

After surface hardening by both regimes, phase changes occur in the metal in the solid state. The parameters of the second regime (I = 120 A, Vp = 220 mm/min) have limit val- ues, i.e., envisage that the surface layer should be heated to the melting point, although no signs of definite melting are observed. There is a gradual transformation over the thick- ness of the surface layer from a zone of complete austenitiza- tion and subsequent hardening to the initial structure of the base metal. The inhomogeneity of the carbon distribution in the austenite is inherited by the martensite and propagates into the depth of the hardened layer, where there are incom- pletely dissolved carbide laminas of the former pearlite. This determines the morphology of the martensite and its substruc- ture and composition formed as a result of rapid cooling due to the high thermal conductivity of the metal. The inhomo- geneity of the structure is determined by the initial state of the metal and the energy and time parameters of the hardening

HV

110( 900

700

500

300

1oo

h, Jam a

1100 12 2

300

100

45 40Kh U8 KhVG

b 2

2

45 40Kh U8 KhVG

Fig. !. Microhardness and thickness of the hardened layer in the studied steels after volume hardening (0) and after plasma-arc hardening by regimes l ( l )and2(2) .

processes. An increase in the rate of treatment increases the inhomogeneity of the structure and decreases the thickness and increases the hardness of the strengthened layer (Fig. 1).

A layerwise electron-microscopic analysis has shown that the structure of hardened layers in steels U8 and KhVG is represented by twinned lamellar martensite. In hardened lay- ers of steels 45 and 40Kh, especially on the surface, we ob- served both lath and lamellar twinned martensite (Fig. 2a), which is rare in volume-hardened steels. Lamellar twinned martensite is probably formed at sites of former pearlite colo- nies, where an elevated carbon concentration is retained due

384 D.S. Stavrev et al.

TABLE 2

CA Cu h, lam Aret, % K 15, rad %

Surface 36 1.23 0.95 0.02706 400 20 0.85 0.70 Fe(Cr)3C, Cr23C 6 0.02157 700 - - 0.30 The same 0.01700

Note. The results are presented for steel KhVG treated by regime 1. Notation. h) distance from the surface; A m ) mount of retained aust~te; CA, CM) carbon contents in austenite and martensite, respectively;K) car- bides; 15) physical broadening.

to the short hold at high temperatures. This determines the special features of the structure of microvolumes with a high carbon content (relative to microvolumes where the carbon content is moderate) in subsequent rapid cooling, in which the martensitic transformation occurs by a mechanism typical for high-carbon steels. Fine carbide particles incompletely dissolved during the rapid heating are retained in the formed martensite crystals and impede or orient crystal growth along eementite particles of former pearlite colonies. In addition, undissolved carbide particles hamper the growth of austenite grains until a high surface temperature (almost equal to Tnzlt ) is attained. In lath martensite the size of the laths and their orientation often correspond to the size and orientation offer- rite lameilas in a pearlite colony, which is a sign of inheri- tance in the phase transformations. Another possible site of formation of lath martensite is former ferrite grains, which give way to austenite with a lower carbon concentration. It can also be assumed that in the middle of these carbon-de- pleted volumes low-carbon ferrite or a distinctive troostite can be retained. Lath martensite is also formed along the pe- riphery of former ferrite grains.

The inhomogeneity of the structure increases in all the studied steels with the distance from the surface. At a dis- tance of 300 - 700 tam from the surface, steel KhVG exhibits undissolved carbides of the types Fe3C, Fe(Cr)3C , and Cr23C 6, and the martensite crystals are finer. A distinctive martensite-troostite structure followed by a troostite-ferrite structure with a well-defined inheritance with respect to the initial structure are observed (Fig. 2c). In addition, former pearlite grains are retained (in steels 45 and 40Kh); in them, due to the large number of undissolved carbide lameilas, the martensite crystals are elongated and their sizes are compara-

ble with the thickness of the ferrite larnellas from pearlite (Fig. 2b). This occurs due to the higher rate of the ct ~ "t transformation in heating and of the martensitic transforma- tion in cooling compared with the dissolution of carbides and the homogenization of austenite. As a result of the structural inhomogeneity different microvolumes have different proper- ties.

On the whole, the special features of the structure after plasma-arc hardening provide an increase in the microhard- ness compared to volume hardening, and it is greater the more carbon the steel contains (see Fig. 1). It should be noted that the difference in the mean values of the mierohardness after plasma-arc treatment in accordance with regimes 1 and 2 is slight. The structural differences over the thickness of the strengthened zone manifest themselves in the scattering of the microhardness values caused by the microstructural inho- mogeneity. Homogenizing is rather complete in the surface layers of hardened steels 45 and 40Kh, where the temperature is high, and the difference between the maximum and mini- mum hardnesses is low. At a distance of 400 - 500 tam from the surface this difference increases due to the inhomogeneity of the structure.

The results of the microscopic analysis are confirmed by the x-ray diffraction method. Different amounts of retained austenite were recorded over the thickness of the hardened zone (Tables 2 and 3). With increase in the distance from the surface the amount of retained austenite and carbon dissolved in it decreases. The carbides dissolve completely in the austenite in the heated surface layer and stabilize it in cool- ing. These process are weak at a distance of 300 - 700 tam from the surface, where a large amount of the carbide phase is retained.

The carbon content in the retained austenite on the sur- face of the hardened zone after treatment by either of the two regimes exceeds its mean content in the steel, which indicates that carbon is distributed nonuniformly in austenite after rapid and brief heating. The large amount of retained austenite can be explained by a high content of carbon and alloying elements that decrease the point of the martensitic transformation.

In specimens of steel KhVG subjected to plasma-arc treatment the amount of retained austenite is considerably higher (36%) than after volume hardening (5%) by a standard

TABLE 3

Steel Treatment Am, % cA cu

% K 13, tad

KhVG Hardening from 850C in oil 5 Surface treatment by regime I 36 Surface treatment by regime 2 33

1.23 1.26

0.63 Traces 0.02106 0.95 Cr23C6, Fe(CrhC 0.02706 0.95 Cr23C6, Fc(Cr)3C 0.02833

40Kh Hardening from 8500C in water Surface treatment by regime 1 Surface treatment by regime 2

Tl'aCC$ - 0 .35

- 0 .33 -0 .50

- 0 .35

The notation is as m Table 2.

0,01305

0.01608

0.01783

Effect of Plasma-Arc Treatment on Structural Transformations of Carbon and Alloyed Steels 385

regime (a comparatively low temperature of heating for hard- ening and a low cooling rate). After plasma-arc treatment by regime 1 the structure of steel 40Kh exhibits traces of re- tained austenite.

The profile of the martensite line (200) changes depend- ing on the kind of heat treatment used and the chosen regime of surface hardening. Directly after a treatment by different regimes steel KhVG possessed a certain amount of tempered martensite (from 5 to 15%), and the initial highly tetragonal rnartensite occupied the main part of the volume (Tables 2 and 3). The martensite doublet component (002) has been re- corded for a wide range of reflection angles, which is an ob- vious sign of high inhomogeneity of the a-solid solution (Ta- ble 2) formed from the originally inhomogeneous austenite. The change in the profile of the martensite line over the thick- ness of the hardened layer indicates that the mean concentra- tion of carbon in the martensite decreases.

The results of an investigation of the wear resistance of steels with different carbon contents agree with the results of the microsla'uctural analysis of the specimens after plasma-arc treatment. The wear resistance increased due to the formation of a structure with considerably bbroken-up austenite grains, laths and laminas of martensite, and the presence of fine car- bides of the types Fe3C, Fe(Cr)3C, Crz3C 6. It follows from Fig. 3 that the wear intensity depends on the carbon content in the steel and that it is much lower after plasma-arc harden- ing than after volume hardening. The effect is the most pro- nounced in specimens of structural steel 40Kh.

CONCLUSIONS

1. Plasma-arc treatment increases the hardness and wear resistance of all the steels studied, especially steel 40Kh.

2. A preliminary heat treatment (toughening) is recom- mended in order to provide high toughness and ductility of the core and eliminate cracking after plasma-arc treatment, make the hardened layer more homogeneous, and provide a

AM, mg

,L

36- 0 32-

28-

24-

16

12-

8 t

0

U8

0

KhVG

Fig. 3. Wear resistance of the studied steels (AM is the loss of mass) af- ter volume hardening (0) and after plasma-arc treatment by regimes ! (!) and 2 (2).

smoother variation of the properties from the surface to the COre,

3. The surface hardness and wear resistance of structural steels subjected to plasma-arc surface treatment are improved due to the dispersity and local inhomogeneity of the marten- site structure (both lath and lameilar) with a varying carbon content and presence of carbide particles undissolved in the heating process.

4. The surface hardness and wear resistance of tool steels after plasma-arc treatment increase for the same reasons and also due to the presence of special carbides, the increase in the proportion of lath martensite, the higher carbon content in the martensite, and the presence of retained austenite in the surface layer, which decreases cracking and chipping in nm- ning-in.

REFERENCES

1. M.A. Sha'emel' and L. M. Kaputkina, "X-ray diffraction analysis of polycrystals of carbon martensite," Fiz. Met. Metalloved., 32(51), 991 - 997 (1971).