fracture behavior of intercritically treated complex structure in medium-carbon 6Ni steel
- PDF / 3,048,780 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 110 Downloads / 156 Views
I.
INTRODUCTION
L o w carbon complex steels which are intercritically treated in (c~ + 7) region can be classified into two types. One type is (5-9) Ni steels for low temperature uses. These steels are able to be intercriticaUy treated in the range of 550 ~ to 670 ~ due to relatively sufficient Ni additions lowering the intercritical region. For the development of low temperature toughness, the effective grain size of matrix (ferrite and/or tempered martensite) is refined through the fine distribution of thermally, mechanically stable austenite. The other type is Mn-Si dual phase steels for high strength/ductility automotive uses. After these steels are intercritically treated in the range of 750 ~ to 850 ~ the complex structure having 10 to 20 pct martensite and small amount of retained austenite in ferrite can be obtained at room temperature. For high strength/ductility, the fine distribution of martensite which decreases the effective ferrite grain size (i.e., mean ferrite path) is desired. 2'3 In medium carbon steels intercritically treated, however, since the excess amount of quite coarse, blocky martensite is formed, the toughness and ductility are severely decreased. 4-5 In this study, the fracture behavior of complex structure in 6Ni-0.3C steel in which the intercritical treatment in the low temperature region is possible and thus the amount and distribution of the fibrous martensite obtained from the initial microstructure of coarse-grained martensite can be effectively controlled has been investigated.
II.
EXPERIMENTAL PROCEDURE
About 20 kg experimental alloy ingot 100 z 100 x 250 mm was prepared by means of vacuum induction melting process. The alloy composition is 5.75Ni, 0.3lC, n,-
m
Z u.I
75
I-U
a.
50
n
r9
r
\
25
0
A
I
I
|
I
200
250
300
350
400
MICROVICKERS
HARDNESS(Hv)
Fig. 7--MLcrowckers hardness to impact toughness relation.
Therefore, as the amount of martensite increases and the martensite coarsens, the lower energy fracture exhibiting the occurrence of the smaller plastic deformation tends to occur along the ferrite-coarser martensite interfaces at the grain boundaries. At the present time, whether the self-weakening of those interfaces with increasing the intercritical treating temperature and time occurs or not is uncertain.
V.
CONCLUSIONS
1. The complex structure intercritically treated in the range of 630 ~ to 670 ~ which has the fibrous martensite is obtained in 6Ni-0.3C steel.
VOLUME 18A, SEPTEMBER 1 9 8 7 - - 1591
2. The amount of martensite increases and the martensite coarsens with increasing the temperature and time of the intercritical treatment. 3. The hardness increases, but the impact toughness decreases with increasing the amount of martensite. 4. As the impact toughness decreases, the fracture along the prior austenite grain boundaries becomes predominanl and the fracture mode is changed from dimple type to low energy tear type. 5. As the amount of martensite increases and the martensite coarsens, the susceptibility of low energy fracture alon
Data Loading...
