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Structure and residual stresses in the surface layer of borided tool steels
Journal
of the Less-Common
Metals,
STRUCTURE AND RESIDUAL OF BORIDED TOOL STEELS*
D. TENEVA Central
117
(1986)
369
STRESSES
- 373
369
IN THE SURFACE
LAYER
and M. JAPRAKOVA
Institute
of Mechanical
Engineering,
Sofia
(Bulgaria)
Summary
In this paper the significance of boriding in extending the lifetime of tools performing under conditions of friction and high surface loads, at normal and high temperatures is studied. Boriding conditions and melt compositions are given, for the heat treatments necessary in order to improve the mechanical and technological properties of the basic tool material. The optimum heat treatment conditions are established. The structure of the borided surface layer obtained on several tool steels, frequently used in practice, has been investigated. The residual stresses in the surface layer are determined. Results of microstructural and X-ray diffraction analysis are presented.
1. Introduction
Boriding has increasingly been applied in recent years to improve the life of parts and tools. Layers are formed on the surfaces of products by diffusion saturation with boron. Very high hardness, wear and heat resistance are features of these layers. This permits the application of boriding in the production of tools. In particular, tools for cold and hot forming of steels, which also suffer impact loads when in use. However, owing to boriding changes in the chemical composition and structure of the surface take place, usually accompanied by volume changes. It is therefore of interest to establish the stresses in the surface layers of borided tools, and especially those being borided and heat treated.
2. Experimental
details
Samples made of YlOA Steel (GOST 1435-74), X12M Steel and 5XHM Steel (GOST 5950-73) were studied. The chemical compositions of the steels are presented in Table 1. The samples were borided in an industrial crucible *Paper presented at the 8th Nitrides and Related Compounds,
International Symposium on Boron, Tbilisi, October 8 - 12, 1984.
Elsevier
Sequoia/Printed
Borides,
Carbides,
in The Netherlands
370 TABLE
1
Chemical
Steel
composition
of the steels investigated
Chemical composition (%) c
Mn
YlOA
0.95-1.04
X12M 5XHM
1.45 - 1.65 0.50 -0.60
TABLE
2
Boriding Steel
YlOA
si
cr
0.15-0.30
0.15-0.35
kO.20
0.15 - 0.40 0.50 -0.80
0.15 -0.35 0.15 -0.35
11.0 0.50
and heat treatment
MO
V
Ni
-
-
-
0.15 -0.30 -
1.40
- 12.5 0.40 -0.60 - 0.80 0.15 -0.30
_1.80
conditions
Boriding conditions
Hardening conditions
Tempering
conditions
Temperature (“C)
Time (h)
Temperature (“C)
Cooling medium
Temperature (“C)
Time (h)
850
3
780
In oil
170
1
through water X12M
990
3
980
Oil
450
1.5
BXHM
890
3
860
Oil
190
1
electric resistance furnace, in a melt containing borax and silica-calcium. The conditions for boriding are given in Table 2. Boriding could be the last finishing operation for products with no specific requirements as to improved mechanical properties for their cores; the layer itself performs at a relatively low specific pressure. Forming tools, however, perform hard duties under friction, high surface loads and impacts. In order to withstand these conditions they need to be heat treated, hardened and tempered to improve the mechanical properties of the core and to provide appropriate hardness in the bulk material. It is best to carry out the hardening directly from the boriding bath in order to eliminate repeated heating. After boriding one group of samples was air cooled, a second group of samples was hardened and the third group of samples was both hardened and tempered. Hardening and tempering conditions are presented in Table 2. The structure and phase composition of the borided steels was invesmicrohardness and X-ray diffraction analysis tigated by microstructural, using Co Ko radiation. Figures 1 - 3 show the microstructure of the borided steels with the inden~tions from the microh~dness measured 1850 - 1530 IN 0.1. The layer is white and textured. Radiation with Co Kcu (normal picture, tilt angle $ = 0” for crystallographic planes (Ml), coinciding with the surfaces of the sample) revealed a preferred orientation for FeB in the direction of the crystallographic plane (002), d = 1.48 A, and for FezB in the
371
Fig. 1. Microstructure
of YlOA Steel, borided
Fig. 2. Microstructure
of 5XHM Steel,
borided
Fig. 3. Microstructure
of X12M Steel, borided
at 890 “C for 3 h. (Magnification, at 890 “C for 3 h. (Magnification,
at 990 “C for 3 h. (Magnification,
150X.) 150x.)
150x.)
direction of the crystallographic plane (002), d = 2.12 8, i.e. the c axis for the respective type of lattice is perpendicular to the surface of the sample, and this is in agreement with the results reported in ref. 1. This is explained by the diffusion anisotropy of the formation of the boride layer. In fact, it is quite evident from Figs. 1 - 3 that the layer has a tooth-like structure; the teeth are perpendicular to the surface, FeB boride is at the surface and looks slightly darker even after normal etching. The high carbon content of the steels investigated results in “smoothing” of the teeth. Underneath the borides, the so-called boron cementite Fe,(B,sC,,) is formed; it is also textured but differs in orientation.
372
In all samples the presence of FeB and Fe,B is established, it is thus safe to conclude that the layer consists predominantly of Fe,B with a narrow strip of FeB on the surface, which is discontinuous in some places. This is favourable given the conditions in which the treated tools are intended to be used. X-ray layer analysis confirmed the presence of Fe3(Bo.sC&) and Fe&B&%. For X-ray determination of the macrostresses Cr Kcu radiation was used. The analysis was made using the (212) reflection, d = 1.17 8, for FeB, and the (123)/(330) reflection, d = 1.20 A, for Fe,B. The location of the reflections is determined from the position of the Ka,,, doublet. The elasticity moduli (X-ray) for FeB and Fe,B are taken from ref. 2. The results obtained are presented in Table 3. TABLE Residual
3 stresses
in the surface
layers of tool steels depending
on type of treatment
Steel grade
Type of treatment
UP W’a) FBI
YlOA
Boriding Boriding Boriding
+ hardening + hardening
Boriding Boriding Boriding
+ hardening + hardening
Boriding Boriding Boriding
+ hardening + hardening
5XHM
X12M
+ tempering
-784 -933
+ tempering
-70 -91 -235
+ tempering
-219 -338 -629
The X-ray diffraction measurement of the stresses in the surface layer was only possible for FeB. It is quite evident from our data that compressive stresses are obtained on the surfaces of all samples investigated. This is explained by the formation of the borides FeB and Fe,B, with a higher specific volume, in the surface layer. After tempering a tendency is observed for increase in the compressive residual stresses. These data are supported by the data reported earlier for 5XHM Steel [ 31. Industrial tests of boride forming tools show an improvement by a factor of 1.5 - 5 in their lifetime as compared with that of conventionally heat treated tools.
3, Conclusions 1. To obtain the optimum properties the borided layer should consist predominantly of FezB with thin surface layer of FeB; the FeB layer is more brittle but provides higher wear and heat resistance.
373
2. As a result of boriding and heat treatment of tool steels compressive residual stresses appear on their surfaces, increasing their resistance to wear and to plastic deformation. 3. Tempering of borided and hardened steel results in an increase in the compressive residual stresses in the boride layer.
References 1 M. Deger, M. Riehle and W. Schatt, Neue Hiitte, 17 (6) (1972) 341 2 E. Tokashi and K. Mamoru, J. Sot. Muter. Sci. Jpn., 32 (352) (1983) 3 D. Teneva and M. Japrakova, unpublished.
- 347. 114 - 120.
of the Less-Common
Metals,
STRUCTURE AND RESIDUAL OF BORIDED TOOL STEELS*
D. TENEVA Central
117
(1986)
369
STRESSES
- 373
369
IN THE SURFACE
LAYER
and M. JAPRAKOVA
Institute
of Mechanical
Engineering,
Sofia
(Bulgaria)
Summary
In this paper the significance of boriding in extending the lifetime of tools performing under conditions of friction and high surface loads, at normal and high temperatures is studied. Boriding conditions and melt compositions are given, for the heat treatments necessary in order to improve the mechanical and technological properties of the basic tool material. The optimum heat treatment conditions are established. The structure of the borided surface layer obtained on several tool steels, frequently used in practice, has been investigated. The residual stresses in the surface layer are determined. Results of microstructural and X-ray diffraction analysis are presented.
1. Introduction
Boriding has increasingly been applied in recent years to improve the life of parts and tools. Layers are formed on the surfaces of products by diffusion saturation with boron. Very high hardness, wear and heat resistance are features of these layers. This permits the application of boriding in the production of tools. In particular, tools for cold and hot forming of steels, which also suffer impact loads when in use. However, owing to boriding changes in the chemical composition and structure of the surface take place, usually accompanied by volume changes. It is therefore of interest to establish the stresses in the surface layers of borided tools, and especially those being borided and heat treated.
2. Experimental
details
Samples made of YlOA Steel (GOST 1435-74), X12M Steel and 5XHM Steel (GOST 5950-73) were studied. The chemical compositions of the steels are presented in Table 1. The samples were borided in an industrial crucible *Paper presented at the 8th Nitrides and Related Compounds,
International Symposium on Boron, Tbilisi, October 8 - 12, 1984.
Elsevier
Sequoia/Printed
Borides,
Carbides,
in The Netherlands
370 TABLE
1
Chemical
Steel
composition
of the steels investigated
Chemical composition (%) c
Mn
YlOA
0.95-1.04
X12M 5XHM
1.45 - 1.65 0.50 -0.60
TABLE
2
Boriding Steel
YlOA
si
cr
0.15-0.30
0.15-0.35
kO.20
0.15 - 0.40 0.50 -0.80
0.15 -0.35 0.15 -0.35
11.0 0.50
and heat treatment
MO
V
Ni
-
-
-
0.15 -0.30 -
1.40
- 12.5 0.40 -0.60 - 0.80 0.15 -0.30
_1.80
conditions
Boriding conditions
Hardening conditions
Tempering
conditions
Temperature (“C)
Time (h)
Temperature (“C)
Cooling medium
Temperature (“C)
Time (h)
850
3
780
In oil
170
1
through water X12M
990
3
980
Oil
450
1.5
BXHM
890
3
860
Oil
190
1
electric resistance furnace, in a melt containing borax and silica-calcium. The conditions for boriding are given in Table 2. Boriding could be the last finishing operation for products with no specific requirements as to improved mechanical properties for their cores; the layer itself performs at a relatively low specific pressure. Forming tools, however, perform hard duties under friction, high surface loads and impacts. In order to withstand these conditions they need to be heat treated, hardened and tempered to improve the mechanical properties of the core and to provide appropriate hardness in the bulk material. It is best to carry out the hardening directly from the boriding bath in order to eliminate repeated heating. After boriding one group of samples was air cooled, a second group of samples was hardened and the third group of samples was both hardened and tempered. Hardening and tempering conditions are presented in Table 2. The structure and phase composition of the borided steels was invesmicrohardness and X-ray diffraction analysis tigated by microstructural, using Co Ko radiation. Figures 1 - 3 show the microstructure of the borided steels with the inden~tions from the microh~dness measured 1850 - 1530 IN 0.1. The layer is white and textured. Radiation with Co Kcu (normal picture, tilt angle $ = 0” for crystallographic planes (Ml), coinciding with the surfaces of the sample) revealed a preferred orientation for FeB in the direction of the crystallographic plane (002), d = 1.48 A, and for FezB in the
371
Fig. 1. Microstructure
of YlOA Steel, borided
Fig. 2. Microstructure
of 5XHM Steel,
borided
Fig. 3. Microstructure
of X12M Steel, borided
at 890 “C for 3 h. (Magnification, at 890 “C for 3 h. (Magnification,
at 990 “C for 3 h. (Magnification,
150X.) 150x.)
150x.)
direction of the crystallographic plane (002), d = 2.12 8, i.e. the c axis for the respective type of lattice is perpendicular to the surface of the sample, and this is in agreement with the results reported in ref. 1. This is explained by the diffusion anisotropy of the formation of the boride layer. In fact, it is quite evident from Figs. 1 - 3 that the layer has a tooth-like structure; the teeth are perpendicular to the surface, FeB boride is at the surface and looks slightly darker even after normal etching. The high carbon content of the steels investigated results in “smoothing” of the teeth. Underneath the borides, the so-called boron cementite Fe,(B,sC,,) is formed; it is also textured but differs in orientation.
372
In all samples the presence of FeB and Fe,B is established, it is thus safe to conclude that the layer consists predominantly of Fe,B with a narrow strip of FeB on the surface, which is discontinuous in some places. This is favourable given the conditions in which the treated tools are intended to be used. X-ray layer analysis confirmed the presence of Fe3(Bo.sC&) and Fe&B&%. For X-ray determination of the macrostresses Cr Kcu radiation was used. The analysis was made using the (212) reflection, d = 1.17 8, for FeB, and the (123)/(330) reflection, d = 1.20 A, for Fe,B. The location of the reflections is determined from the position of the Ka,,, doublet. The elasticity moduli (X-ray) for FeB and Fe,B are taken from ref. 2. The results obtained are presented in Table 3. TABLE Residual
3 stresses
in the surface
layers of tool steels depending
on type of treatment
Steel grade
Type of treatment
UP W’a) FBI
YlOA
Boriding Boriding Boriding
+ hardening + hardening
Boriding Boriding Boriding
+ hardening + hardening
Boriding Boriding Boriding
+ hardening + hardening
5XHM
X12M
+ tempering
-784 -933
+ tempering
-70 -91 -235
+ tempering
-219 -338 -629
The X-ray diffraction measurement of the stresses in the surface layer was only possible for FeB. It is quite evident from our data that compressive stresses are obtained on the surfaces of all samples investigated. This is explained by the formation of the borides FeB and Fe,B, with a higher specific volume, in the surface layer. After tempering a tendency is observed for increase in the compressive residual stresses. These data are supported by the data reported earlier for 5XHM Steel [ 31. Industrial tests of boride forming tools show an improvement by a factor of 1.5 - 5 in their lifetime as compared with that of conventionally heat treated tools.
3, Conclusions 1. To obtain the optimum properties the borided layer should consist predominantly of FezB with thin surface layer of FeB; the FeB layer is more brittle but provides higher wear and heat resistance.
373
2. As a result of boriding and heat treatment of tool steels compressive residual stresses appear on their surfaces, increasing their resistance to wear and to plastic deformation. 3. Tempering of borided and hardened steel results in an increase in the compressive residual stresses in the boride layer.
References 1 M. Deger, M. Riehle and W. Schatt, Neue Hiitte, 17 (6) (1972) 341 2 E. Tokashi and K. Mamoru, J. Sot. Muter. Sci. Jpn., 32 (352) (1983) 3 D. Teneva and M. Japrakova, unpublished.
- 347. 114 - 120.