Effect of high-temperature heating on chemical changes in M 7 C 3 carbides of AISI D2 tool steel

The paper presents the study results obtained on 16H12MF/NC11LV/D2 tool steel. The main purpose of the study was to establish the chemical composition of the coarse primary M 7 C 3 carbides occurring in that steel after a standard soft annealing. The effect of high-temperature annealing in the air atmosphere was investigated. The study was limited to the decarburized layer, on the distance of about 0.4 mm from the surface, of hardened steel after annealing and austenitizing at 1150 ºC in the air atmosphere for the periods of 30 and 90 minutes. It was found that the coarse primary M 7 C 3 carbides of the annealed D2 tool steel differ significantly as to the contents of Cr, Mo, and V, and in the most degree to the contents of chromium and molybdenum. The average concentration of chromium rises successively with the growth of austenitizing time. On the other hand, the concentrations of molybdenum and vanadium are lower after 30 minutes of austenitizing than their concentrations in the similar carbides of the annealed steel. Prolongation of the austenitizing time up to 90 minutes results in the increase of molybdenum and vanadium contents in the coarse carbides. It was found that austenitizing of AISI D2 tool steel at the temperature of 1150 ºC in the air atmosphere leads to precipitation of the second phase, brighter in the BSE_Z pictures, in relation to the matrix of carbides, with the amount and magnitude being higher with the annealing time.

Decrease of chromium content in M 7 C 3 carbides as the function of temperature 950-1150 ºC and austenitizing time up to 30 min occurs also in tool steels of type 2 % C and 12 % Cr with the additives of such elements like tungsten and vanadium . Apart from this in these works it was also revealed that with the growth of temperature and time up to 30 min, the content of tungsten in carbides also decreases with some slight increase of vanadium. It results from Haberling and Schruff [31] also that with the increase of austenitizing temperature of EN-X155CrVMo121 steel from 1030-1080 ºC the chromium content in the M 7 C 3 carbides decreases along with the rise of vanadium concentration, with practically no change in molybdenum content observed.
The phenomenon of fluctuation of chemical composition in carbides during austenitizing occurs also in M 6 C carbides of high speed steel [32], in M 23 C 6 carbides of corrosion resistant steels [33], as well as cementite (Fe,Cr) 3 C carbides of bead steels [34,35].
Many investigators were dealing with the study of structure and chemical composition of chromium-iron carbides [36][37][38][39][40][41][42][43][44][45][46][47][48][49]. The main aim of this work, with the results presented herewith, was to establish the behavior of chromium, molybdenum and vanadium in M 7 C 3 carbides occurring in NC11LV (160H12MF) steel austenitized at 1150 ºC. The chemical composition of the carbides in these steels after softening annealing was also performed. It was also determined the austenitizing time of this steel in the air atmosphere on the chemical composition and displacement of C, Cr, Mo, and V in the big primary M 7 C 3 carbides occurring in the de-carburized layer. Review of the subject literature done shows there is no elaborations concerning presented subject.

MATERIAL AND STUDY METHOD
The studies were performed on the NC11LV/D2 steel of chemical composition given in Table 1. These data were obtained by the method presented below. Chemical composition with the contents of elements was determined based on the analysis of 6 samples of the softening annealed steel of different rods of diameter 14.5 mm. The rods were coming from the same heat. Chemical composition of the D2 steel was determined on the emission optical specterometer SPECTROLAB TYPE 05 S/N 45/263. Samples for the studies were heat treated by austenitizing at 1150 ºC and hardening in oil. Austenitizing annealing was performed in times 10, 30, and 90 minutes in the air atmosphere. The temperature regulation accuracy was ±5 ºC.
The following studies were carried out: (1) the metallographic microscopic studies were performed on the microscope Neophot 2 and Epityp 2. The microsections were prepared mechanically on the abrasive papers of graininess 150-2500, using polishing by means of aqueous Al 2 O 3 suspension afterwards. To reveal microstructure of the steel in the softening annealed state the microsections were etched in 4 % HNO 3 diluted with C 2 H 5 OH, whereas that hardened after austenitizing at 1150 ºC and time of 30 min by means of a reagent of composition: 1 g picric acid, 5 ml HCl, and 100 ml C 2 H 5 OH; (2) investigation of microhardness of coarse primary carbides was done by means of Hanemann microhardness tester mph100 and the microscope Epityp 2 using load of 10 G; (3) the thin foils studies of samples of other steel of type type 2% C and 12% Cr with the additives of tungsten and vanadium, hardened after austenitizing at 1150 ºC and time of 90 min were performed by means of a transmission electron microscope BS613 and BS540 of TESLA Co. Thin foils were prepared by using initial thinning on the abrasive papers of graininess 300-800, with electrolytic thinning performed afterwards by means of TUNEPOL apparatus of STUERS Co. 10 % HClO 4 solution in C 2 H 5 OH of temperature 5 ºC using 40 V voltage was used for the electrolytic thinning. The structure photographs of thin foils were done at 100 kV voltage; (4) the volume fraction of carbides was determined by quantitative metallography, i.e. by a point/pointwise method using a network of 169 notches of symmetric distribution.
Counting of a number of the network notch hits in the carbide was carried out on the microphotographs. The required/needed number of network applications/taking naps was determined from the relationship: by assuming as follows [50]: ̶  = 0.1 as an absolute error and the probability (1α) = 0.9, with ̶ t α =1.645 as the value taken from the tables of a standard distribution ̶ z -number of network applications ̶ P w -volume fraction of carbides (determined based on the observations). It was established that the necessary number of network applications equals 5 for the annealed D2 tool steel and 14 for the hardened steel after austenitizing at 1150 ºC and time 30 min. In fact 10 and 15 network applications were used, respectively; therefore more applications were used; (5) the steel hardness under annealed state was determined by Brinell method, whereas the hardened steel by a Rockwell method using C scale; from the surface plane) after annealing in time of 30 and 90 minutes, respectively, were performed. They were carried out by means of French made X-ray microanalyzer SEMPROBE Su30 of CAMECA Co. equipped with two X-ray spectrometers WDS and a spectrometer EDS VOYAGER 3 of NORMAN Instruments (USA made). Investigation of surface distribution of elements were performed by means of WDS spectrometer. The accelerating voltage for electron excitation beam was V = 15 kV, and the beam current was I = 15 nA. The studies of linear displacement of elements were performed using EDS spectrometer at the voltage V = 15 kV, and the beam current was I = 580 pA; (7) the chemical composition studies of coarse primary carbides were performed by X-ray microanalysis method. They were carried out by means of microanalyser CAMECA S 30 equipped with 2 X-ray spectrometers with a dispersion of length wave (WDS). The 20-second measurements of intensity of characteristic X-radiation of carbon, chromium, molybdenum, and iron were performed on each selected carbide at the accelerating voltage V = 15 kV and the beam current I = 20 nA. Diamond and spectrally pure metals (chromium, molybdenum, vanadium) served for the pattern of analyzed elements. A standard/commercial correction programme, served by CAMECA Co., based on Pouchou and Pichoir model [20], was used to determine element concentrations in the studied samples. The effect of contamination of a specimen surface by carbon during the analysis was corrected by the method described in the work [31].

1. Microstructure and hardness of D2 tool steel softening annealed and hardened after austenitizing at 1150 ºC
The studied D2 tool steel in the annealed state has the hardness of 201 HB and the structure composed of primary M 7 C 3 carbides, and the secondary ones distributed in the ferritic chromium-molybdenum-vanadium matrix (Fig. 1). The carbide contribution determined by the point/pointwise method equals 24.7 ±2 % .
The shape of coarse primary carbides is usually non-regular and on the longitudinal rod cross-section some of them possess the shape close-to-rectangular placed in the direction corresponding with the direction of force action during plastic treatment. The primary carbides of smaller dimensions and the secondary carbides have the shape close-to-spheroidal. The average microhardness of the coarse carbides, determined based on the measurements of 12 carbides, equals 1384 ±103 HV0.01.
The hardness of steel hardened after austenitizing at 1150 ºC for 30 min period equals 37.9 HRC. With the increasing time up to 90 minutes it slightly rises up to 38.6 HRC. The growth of hardness after high-temperature austenitizing and longer times was noted also in other hardened this type of steels [3,5,7,8].
The matrix of the studied steel hardened after austenitizing for 10 to 90 minutes is composed of austenite with the primary carbides appearing on its background and numerous secondary carbides which did not dissolve during the austenitizing.
The steel hardened after austenitizing for 30 min contains 10.4 ±1.2 vol % of carbides. Microhardness of coarse primary carbides in the hardened steel is higher than the microhardness of similar ones occurring in the annealed steel, e.g. the microhardness in the hardened steel after austenitizing for 30 min equals 1522 ±39 HV0.01. The reasons of higher microhardness of carbides are the changes in chemical composition and arising in them, during cooling the steel after austenitizing annealing, different kinds of defects. The occurrence of that phenomenon was found/noticed in the studies of M 7 C 3 carbides in other steel of type 2 % C and 12% Cr with the additives of tungsten (1.32 %) and vanadium (0.31 %). The studies were performed on the transmission electron microscopes TESLA BS613 and BS40 using a thin foil technique. Some examples of the occurrence of defects in M 7 C 3 carbides in the higher mentioned steel hardened after austenitizing at 1150 ºC for 90 minutes, are presented in Fig. 2 (a-d).

International Letters of Chemistry, Physics and Astronomy Vol. 36
The structure of non-carburized steel on the distance of 0.4 mm from the sample surface of steel hardened after austenitizing for 30 min in the air atmosphere is presented in Fig. 3 a, b. The magnification of the pictures given in Fig. 3 are comparable to that one of Fig. 1b. During annealing of the studied steel at 1150 ºC for the period of 30 and 90 minutes, the decarburizing occurs on the depth of about 1 and 1.6 mm, respectively. The chemical composition of the decarburized layer on the distance of about 0.4 mm from the surface of hardened steel after austenitizing for 30 min, determined by means of emission optical spectroscope SPECTROLAB, is presented in Table 2.
The decarburized layer at the analyzed distance has the structure consisting of coarse primary carbides and some small amounts of secondary ones on the background of austenite and, occurring in some places, the small amounts of martensite. Coarse carbides are placed mostly on the borders of austenite grains. High dispersion of the austenite grain magnitude was found with the smallest grains occurring in the areas of increased density/frequency of primary carbides.

2. Surface and linear distributions of chromium, molybdenum, and vanadium in the softening annealed D2 tool steel
It results from Figures 4 and 5 that the contents of carbon, chromium, molybdenum, and vanadium in the primary coarse M 7 C 3 carbides of the annealed steel are considerably higher than those in the matrix. The results of analyses of chemical composition of coarse primary carbides in the annealed D2 tool steel obtained by the method of X-ray microanalysis are given in Table 3.
The results presented in Table 3 indicate that the coarse primary carbides in the annealed D2 tool steel, apart from the earlier/previous high-temperature annealing of the steel in the process of plastic treatment and several hours' annealing in the operation of soft annealing (spheroidizing), differ both as to chromium and molybdenum contents. That means the D2 tool steel heating during the higher mentioned technological operations does not cancel differences in chemical composition of coarse primary carbides, resulting from different chemical composition of dendrite branches arisen during the process of steel crystallization.

Linear distributions of chromium, molybdenum, and vanadium in the decarburized layer of D2 tool steel hardened after austenitizing for 30 minutes in the air atmosphere
It results from the studies carried out using dispersed backscatter electrons that in carbides occurring in the decarburized layer of the hardened D2 tool steel after austenitizing at 1150 ºC the second phase arises in the form of bright areas of irregular shape and relatively uniform displacement (Fig. 6). The amount of this phase increases with the increase of annealing time from 30 up to 90 minutes.

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4. Thermodynamic calculation
A thermodynamic calculation may be made using the chemical composition given in Table 1 as input (Thermo-Calc Software TCFE7 Steels/Fe-alloys database version 7 [52]). Assuming equilibrium after exposure for 30 minutes at 1150 ºC, one may find the following values:

CONCLUSIONS
Based on the studies carried out the following conclusions may be formulated: