Formation of structure and properties during the thermal treatment of powder steels with different carbon-containing components
Improving the physical and mechanical properties of powder steels. Study of the effect of heat treatment on the structure of powder carbon steels with carbon-containing components. Research of heat treatment processes: hardening, tempering and annealing.
| Рубрика | Производство и технологии |
| Вид | статья |
| Язык | английский |
| Дата добавления | 27.02.2021 |
| Размер файла | 1,4 M |
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National university of science and technology
Formation of structure and properties during the thermal treatment of powder steels with different carbon-containing components
Nhu Dam Nguyen
Eremeeva Zh.V.
Lopatin V.Yu.
Moscow, Russia
The effect that thermal treatment has on the formation of the structure of hot stamped powder carbon steels with different carbon containing components (CCCs) is determined. Processes of thermal treatment such as quenching, tempering, and annealing are investigated. The inheritance of the initial structure was observed at all steps of thermal treatment.
Improving the physical-mechanical properties of powder steels can be achieved not only by changing their composition and production practice, but also by directional thermal treatment (TT). The results of TT are affected by factors both inherent to cast steels and specific ones caused by the characteristic properties of the initial materials used and technologies for their processing. The possibility of applying different types of TT is based on the fact that each powder particle is compact metal in which the same transformations take place as in large volumes of metal during heating and cooling. However, the features of the thermodynamic state of powder metal do not allow us completely evaluate its structural changes using the regularities observed for compact cast steels.
The specifics of the formation of the structure and properties of hot deformed powder steels obtained from a charge with different carbon-containing components (CCCs) is related to the features of phase transformations and to the character of structures arising under their effect. These phenomena can proceed both directly in the course of cooling the samples after hot stamping (HS) (then they are similar to high temperature thermomechanical treatment (HTMT)) and during the TT of the materials already cooled after HS. In this work, the effect of a cooling rate after hot stamping on the structure formation of powder steels was determined. Water, oil, or air was used as a cooling medium. In the first case, the effect of the HTMT is achieved with a simultaneous increase in the strength and plasticity of samples containing all types of CCCs and any amount of carbon (Tables 1, 2). As should be expected, the highest properties are characteristic of the steels of eutectoid composition, which were obtained with the application of artificial special lowash carbon (ASLC) and high temperature pitch (HP).
Table 1
Properties of powder steels cooled after HS in various media
|
CCC type in the charge |
Carbon content in steel,% |
Cooling medium |
||||||
|
Air |
* Water |
|||||||
|
Ob, MPa |
S,% |
HRC** |
Ob, MPa |
5,% |
HRC** |
|||
|
(HRB) |
(HRB) |
|||||||
|
PL |
0,5 |
515 |
19 |
20 |
750 |
29 1,53 |
25 1,25 |
|
|
1,46 |
||||||||
|
900 |
19 |
34 |
||||||
|
0,8 |
755 |
16,5 |
18 |
|||||
|
1,20 |
1,15 |
1,89 |
||||||
|
1,2 |
1320 |
5 |
33 |
1125 |
16 |
34 |
||
|
0,85 |
3,2 |
1,03 |
||||||
|
0,5 |
775 |
22 |
20 |
925 1,19 |
37 1,68 |
(185) |
||
|
1,13 |
||||||||
|
1275 |
26 |
28 |
||||||
|
ASLC |
0,8 |
975 |
10 |
22 |
-- |
|||
|
1,31 |
2,6 |
1,27 |
||||||
|
1800 |
18 |
38 |
||||||
|
1,2 |
1510 |
8 |
38 |
|||||
|
1,19 |
2,3 |
1 |
||||||
|
0,5 |
750 |
22 |
22 |
1525 |
31 1,41 |
31 1,41 |
||
|
2,03 |
||||||||
|
1850 |
22 |
43 |
||||||
|
HP |
0,8 |
1125 |
17 |
25 |
1,64 2220 |
1,29 15 |
1,72 55 |
|
|
1,2 |
2004 |
13 |
44 |
|||||
|
1,11 |
1,15 |
1,25 |
The denominator represents the ratio of the values of the parameter during cooling in water and in air.
Steel samples after HS were heated at a rate of 56 K/min to a temperature of 850-875°C; held for 1520 min; and cooled Vcool at a rate of 450-500 (in water), 100-150 (in oil), and 30-45 (in air) K/s. The mechanical properties of steels that contain pencil lead (PL) and ASLC within the charge are improved as Vcool rises (like in cast steels), remaining much higher in the second case. An increase in the cooling rate ofthe samples doped with pyrocarbon (PC) from 150 to allows us to use oil as a cooling liquid. Cooling in air did not allow us to obtain a structure characteristic of 450 K/s has almost no effect on their properties, which hardened steel in any of the cases (Table 2).
Table 2
Effect that the cooling rate during tt has on the structural formation of different regions and properties of hot stamping samples of steel 80p for different CCCS in the charge
|
Cooling medium o/s |
Condition |
CCC type in the charge |
|||||||
|
PL |
ASLC |
HP |
|||||||
|
face; |
center |
face; |
center |
face; |
center |
||||
|
Water 450-500 |
Structure |
M+A+B |
M+A+T |
M+A |
M+A+T |
M+A |
M+A |
||
|
properties |
HRC |
35 |
29 |
42 |
33 |
50 |
38 |
||
|
Ob, MPa |
1575 |
1800 |
2075 |
||||||
|
Batter, 100-150 |
Structure |
M+A+B |
M+A+C |
M+A+T |
M+A+C |
M+A |
M+A |
||
|
properties |
HRC |
27 |
20 |
35 |
33 |
35 |
33 |
||
|
Ob, MPa |
1125 |
1500 |
1750 |
||||||
|
Air, 30-45 |
Structure |
T+P+C |
T+P+F |
P+F+A |
P+F |
B+P+C |
P+F |
||
|
properties |
HRB |
89 |
86 |
92 |
88 |
95 |
90 |
||
|
Ob, MPa |
800 |
1075 |
1325 |
The structure of the samples obtained from the charge containing PL after hot stamping consists of Large lamellar pearlite; upon heating, it transforms into large grain austenite. The tendency of PC to segregate causes the formation of austenite with a different carbon saturation of grains, which results in an intensified growth of martensite crystals during quenching. The coherence of austenite and martensite in ASLC-containing and HP-containing steels is violated because of the appearance of dislocations at the interface of these phases during martensite transformation, and the rapid growth of martensite grains is stopped.
The martensite structure with inclusions of residual austenite and bainite is observed on the surface of samples of the eutectoid composition, which were obtained from the PL-containing charge, after annealing in water. Most martensite needles have a rough surface, which is caused by the precipitation of fine carbide particles of the Fe3C type. In its core, bainite is absent, but an increased amount of fine pearlite appears. The fracture has a brittle transcrystalline river character. Residual graphite inclusions are the centers of cracking. Quenching microcracks were observed in large acicular martensite; they intersect martensite plates or are located in the places of joint with cementite inclusions (Fig. 1). The occurrence of cracks indicates significant quenching stresses.
Fig. 1. Microstructure of PL-containing powder steel after quenching in water. (a) Martensite with carbide precipitations(*1000), (b) fractography of the fracture (*3000), (c) coal replica with a fine perlite structure (*5800)0), and (d) fine structure and carbide precipitates (*105)
Fig. 2. Microstructure of PL containing powder steel after quenching in water. (a) Fine foil with twins of martensite needles(*19000), (b) coal replica with globules of residual austenite (*7200), (c) fractography of the fracture (*1500), and (d) coal replica(*5200)
The surface and core of the HP-containing samples after quenching in water have a martensite structure with austenite inclusions; no microcracks were found. Fine martensite needles of different etching abilities and single carbide particles are mainly residual austenite was also found, and it increased as the quenching temperature increases. The destruction is interparticle and brittle; the fracture is stony. thermal powder carbon steel
In the case of the ASLC-containing samples quenched in water, we observed spear-shaped large martensite needles which were formed in the initial period of transformation and fine needles formed during the following cooling. A significant amount of found, destruction proceeds along the martensite plate, and the fracture is brittle and stony (Fig. 2). It is characterized by pits that are divided into honeycombs; consequently, the pit surface is formed during destruction along the fine dispersed sub interfaces.
During the investigation of fine foils obtained from the samples after HS and TT (followed by cooling in water and containing PC in the charge), large packets of martensite crystals were found. Their apparent section in the foil plane has a uniquiaxial shape. In the case of the present treatment, they completely inherit the substructure of initial austenite. We can see on a replica that the sizes of twin martensite needles are significant. These samples are characterized by brittle destruction. The shape of cementite grains changes during annealing in all cases, which is related to the repacking of iron atoms during the phase transformation. Because cementite has a larger specific volume than ferrite, the generation of vacancies, the rate of which increases under the influence of deformation, as well as the presence of carbon is necessary for repacking.
The fact of a simultaneous increase in strength and plasticity during annealing of the materials seems interesting. This can be explained by the healing of defects arising in the course of the hot recompacting of moldings, the improvement of the quality of fusing on the thus formed contact surfaces, and the decrease in the softening action of pores due to their healing and spheroidization. It is noteworthy that, as the annealing duration increases, the properties of steels with all CCCs improve and the ranking of the carbon- containing components, which is discovered at other steps of obtaining and treating hot deformed steels, remains (namely, HP-ASLC-PC-PL). This indicates that the positive qualities of unconventional CCCs are also inherited at this technological step.
Conclusions
The HTMT effect is achieved during the accelerated cooling of hotdeformed powder steels obtained from charges containing different CCCs. This is related to the austenite structure formed upon heating and characterized by fineness, which is inherited by high quality fine needle martensite. In the course of the thermal treatment of powder steel, the martensite structure is formed only during cooling in water. With the use of oil for these goals, such a structure is revealed only for PC-containing and HP-containing steels. No quenching structure arose after cooling in air.
The characteristics of CCCs affect the formation of the structure and properties of steels during tempering, which manifests itself in the inheritance of their features laid at all previous technological steps. Therefore, the properties of ASLC-containing and HP- containing steels tempered at all temperatures improvedwith respect to the PL-containing materials.
Diffusion annealing of the ASLC-containing and HP-containing samples, which are characterized by the homogeneity and fineness of the structure, leads only to the additional refinement of perlite. Therefore, we can either not perform this operation at all, or we can restrict ourselves to holding for 30 min at 1100°C.
Only prolonged annealing for 2 h at t0 = 1100°C was favorable for the formation of a structure with separate regions of granular perlite for PL-containing samples. A homogeneous structure requires an even longer time for annealing.
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