International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 04, Volume 4 (April 2017) (SPECIAL ISSUE) www.ijirae.com
Study of Tungsten Carbide Behavior in Hot Forging Application
Mr. Siddharth Suhas Revankar1, Mr. Ramesh S Rao2, Dr. Y. S Varadarajan3 1M.Tech Student, Department of I&PE NIE, Mysuru.
2Deputy General Manager, RD&E Material Science Kennametal India Ltd Bengaluru 3Professor, Department of I &PE NIE, Mysuru
Abstract: Hot and warm forging processes subject the dies to severe cyclic thermal and mechanical fatigue subsequently resulting in die failure primarily through wear, plastic deformation and heat checking. This ongoing study aims to reduce these failures by application of tungsten carbide based hard material with cobalt binder (WC-Co) as a die material with steel, joined using shrink fitting process. In the present dissertation work, factors affecting the shrink fitting of carbide pellet and steel casing assembly die was analyzed through design of experiments (DOE) approach. The experimental results were compared with the finite element analysis. The controllable parameters of shrink fit process such as outer diameter of the pellet, inner diameter of the casing with interference, wall thickness of casing were identified and the experiments were designed as per full factorial method. With these details the shrink fitted die is simulated for hot forging conditions using die stress analysis.
Keywords—Shrink fitting,Heat checking.
I. INTRODUCTION
Hot forging dies are normally made of hot work tool steel material, tungsten carbide (WC-Co) being good wear resistant material with high hot hardness strength is not generally used in hot forging application .Since tungsten carbides (WC-Co) die gives a good service life when compared to steel dies and are leading in the cold working processes like wire drawing, upsetting, punching, extrusion etc. Its challenging study to make use of tungsten carbide based hard material with cobalt binder in hot forging application where we have many factors affecting such as temperature, impact loads, thermal and mechanical fatigue etc. The carbide dies when used are normally shrinking fitted to steels so as to give required compressive stresses on carbide.
II. LITERATURE SURVEY
Die costs can constitute up to 30% of the production cost of a part and also affect its profitability directly (die manufacturing cost) and indirectly (repair, press downtime, scrap, rework etc.). Hot and warm forging processes subject the dies to severe thermal and mechanical fatigue due to high pressure and heat transfer between the dies and the workpiece. High cyclic surface temperatures result in thermal softening of the surface layers of the dies, subsequently increasing die wear and susceptibility to heat checking. Conventional methodologies such as nitriding or Boriding result in a significant increase in die surface hardness but have not resulted in substantial improvements in die service life. Recently, a number of advanced grades of proprietary hot-work tool steels have been introduced targeted specifically for warm and hot forging applications. Additionally, the potential for use of ceramic and carbide-based materials in appropriate applications is also under research.
A. SUCCESSFUL APPLICATION OF CERAMIC DIES IN MASS PRODUCTION . There have been cases of successful production application of cermet/ceramic dies in Japan. Nissan Motor Co. is using cermet dies made of MoB (ceramic). The material is powder formed and sintered. In production tests, two die materials were tested on forward extrusion of outer race part under warm forging conditions namely [1]
Fig. 1. Die designs used for application of ceramic inserts [Koitabashi, 1995]
International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 04, Volume 4 (April 2017) (SPECIAL ISSUE) www.ijirae.com
Results stated that MoB cermet die could withstand higher temperature (800°C) than the super alloy. Forgings were also done by using ceramic die inserts in place of H13 tool steel in manufacturing engine valves. Sintered silicon nitride was used to make the die inserts. Two different coining tooling designs were investigated for performance of shrink fit and die life shown in Figure 1. Tests were also conducted to compare the performance of H13 with Zirconia and silicon nitride inserts, laboratory results indicate that there is improvement in die life and better dimension control of the forged parts. Although the exact magnitude of improvement in dies life was not disclosed.[2][3]
III. TYPICAL DIE FAILURE MODES
According to survey the main failure modes of hot forging dies is due to following mechanism ABRASIVE WEAR- 70% FATIGUE (MECHANICAL)- 25% OTHERS- CRACKING, PLASTIC DEFORMATION, THERMAL FATIGUE
A. PROPERTIES OF DIE MATERIAL REQUIRED FOR HOT FORGING APPLICATION THERMAL SHOCK RESISTANCE THERMAL FATIGUE RESISTANCE HIGH TEMPERATURE STRENGTH HIGH WEAR RESISTANCE HIGH TOUGHNESS AND DUCTILITY HIGH MACHINABILITY
B. TUNGSTEN CARBIDE PROPERTIES COMPARED TO STEEL The following is a list of Properties of Tungsten Carbide based hard material with cobalt binder. Different Grades of Tungsten Carbide will differ in Strength, Rigidity, and other Properties, but all Tungsten Carbide Material falls into the basic properties listed below. [4]
Strength - Tungsten carbide has very high strength for a material so hard and rigid. Compressive strength is higher than virtually all cast or forged metals and alloys.
Rigidity - Tungsten carbide compositions range from 2 to 3 times as rigid as steel and 4 to 6 times as rigid as cast iron and brass. Young’s Modulus is up to 530–700 GPa
Impact Resistant - The impact resistance is high. It is in the range of hardened tool steels of lower hardness and compressive strength.
Heat and oxidation resistance - Tungsten-base carbides perform well up to about 5370C in oxidizing atmospheres and to 815°C in non-oxidizing atmospheres
Thermal Conductivity - Tungsten carbide is in the range of twice that of tool steel and carbon steel. Coefficient of Friction-Tungsten carbide compositions exhibit low dry coefficient of friction values as compared to steels. Wear-Resistance - Tungsten carbide wears up to 100 times longer than steel in conditions including abrasion, erosion and
galling. Wear resistance of tungsten carbide is better than that of wear-resistance tool steels. Hot Hardness - With temperature increase to 760°C, tungsten carbide retains much of its room temperature hardness. At
760°C, some grades equal the hardness of steels at room temperature Surface Finishes - Finish of an as-sintered part will be about 1.26 microns. Surface, cylindrical, or internal grinding with
diamond wheel will produce 0.45 microns or better and can produce as low as 0.10 to 0.20 microns. Diamond lapping and honing can produce 0.05 microns and with polishing as low as 0.03 microns
IV. SHRINK FITTING OF CARBIDE AND STEEL
The shrink fitted compound cylinders are used for punching dies at high pressure. Compound cylinders are classified into two types - Thin cylinder & Thick cylinder. For high pressure applications thick cylinders are used. When the magnitude of pressure increases and approaches the yield point of the material used, the thickness of cylinder to resist the load approaches infinity. If internal pressure is equal to the yield strength of the material used, thick cylinder fails. Therefore shrink-fitted compound cylinders are used. The Compound cylinders can used for punching dies at higher pressure closer to the yield stress of the material. Optimally designed compound cylinders have equal maximum hoop stress in both - the inner and outer cylinders. The value of this hoop stress is closer to the value of yield stress of the material used. Such compound cylinders were used in automobile, defensive. The compound cylinder consists of one inner most cylinder which is covered other cylinder by interference fit. The cylinders are manufactured such as to maintain the required interference. Then the inner cylinder (carbide pellet) is cooled to a very low temperature or the outer cylinder (steel casing) is heated to a high temperature depending on the interference value, and then both the cylinders are assembled together. After attaining normal room temperature, the inner cylinder expands or the outer cylinder shrinks, which keeps both the cylinders together due to the interference. This causes compressive stress in inner cylinder (carbide pellet) and tensile stress in outer cylinder (steel casing). These stresses should be within the elastic limits of the material. When such compound cylinders are subjected to internal pressure, the compressive stress in inner cylinder is relieved first, and then tensile stress is developed. The tensile stress in steel casing is further increased due to internal pressure.
International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 04, Volume 4 (April 2017) (SPECIAL ISSUE) www.ijirae.com
Generally the three principal stresses in the cylindrical dies are – circumferential stress, radial stress and longitudinal stress. Out of these, the circumferential stress, also known as hoop stress, is maximum and it is the main cause of failure of the dies. In case of open type of cylindrical dies, the longitudinal stress is absent. When the internal pressure is very high, almost close to its yield point stress, then designing the single cylinder dies would be very bulky and heavy. Then by using compound cylinders, compact and practicable die design can be obtained.[5]
A. VARIOUS FACTORS AFFECTING SHRINK FITTING From the above study we can justify that there are many factors which are affecting the shrink fitting. Hence design of experiments (DOE) approach for understanding the significant factors affecting the shrink fit was conducted using full factorial design in Minitab 17[6]
B. DESIGN OF EXPERIMENTS (DOE) PLAN TABLE-I
According to DOE Plan we can have different combinations of the factors as follows
CARBIDE OD AND STEEL ID WITH INTERFERENCE, WILL HAVE TOTALLY 12 COMBINATIONS 3 DIFFERENT WALL THICKNESS VALUES STEEL OF 3 DIFFERENT GRADES
Total no of combinations for trails will be = 108 Taking number of replicates as 2 we have total of 216 trails, 108 by analytical and other 108 trails by simulation[7] and interaction graphs are plotted using Minitab 17.
18 32 48 47.904 38 0.2 W360 133.13 -479.28 -346.15 253.61 120.48 Similarly for all 108 combinations analytical solution is calculated as shown in TABLE-I
International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 04, Volume 4 (April 2017) (SPECIAL ISSUE) www.ijirae.com
Similarly 108 trails are simulated using Ansys software and values are formulated as shown in TABLE-III
V. MINITAB PLOTS FOR SHRINK FIT ANALYSIS
0
-200
-400
-600
-800
68-0
.468
-0.3
68-0
.268
-0.1
58-0
.458
-0.3
58-0
.258
-0.1
48-0
.448
-0.3
48-0
.248
-0.1
0
-200
-400
-600
-800
382818
COD & Interf * Casing Wall
COD & Interf * Steel Grade
Carbide OD(COD),mm & Interference(%)
Casing Wall * Steel Grade
Casing Wall Thickness,mm
182838
WallCasing
EN24H13W360
GradeSteel
Mea
n of
Com
pres
sive
str
esse
s,Ca
rbid
e ID
,MPa
Interaction Plot for Compressive stresses,Carbide IDFitted Means
Fig. 2. Interaction plot for compressive stress at carbide ID
0
-150
-300
-450
-600
68-0
.468
-0.3
68-0
.268
-0.1
58-0
.458
-0.3
58-0
.258
-0.1
48-0
.448
-0.3
48-0
.248
-0.1
0
-150
-300
-450
-600
382818
COD & Interf * Casing Wall
COD & Interf * Steel Grade
Carbide OD(COD),mm & Interference(%)
Casing Wall * Steel Grade
Casing Wall Thickness,mm
182838
WallCasing
EN24H13W360
GradeSteel
Mea
n of
Com
pres
sive
str
esse
s,Ca
rbid
e O
D,M
Pa
Interaction Plot for Compressive stresses,Carbide ODFitted Means
Fig. 3 Interaction plot for compressive stress at carbide OD
The interaction plot for compressive stresses on carbide ID and OD in figure 2 &3 justify that how compressive stresses vary with interference, casing wall thickness and steel grade. The interference value is directly proportional to the stresses. Casing wall thickness & carbide OD also considerably affects the compressive stresses. As the casing wall thickness increases the compressive stresses developed on carbide increases but tensile stress on steel decreases and hence pre-stress in the carbide can be increased.
International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 04, Volume 4 (April 2017) (SPECIAL ISSUE) www.ijirae.com
Interaction Plot for Tensile stress at steel IDFitted Means
Fig. 4. Interaction plot for tensile stress at steel ID
600
450
300
150
0
68-0
.468-0
.368-0
.268
-0.1
58-0
.458
-0.3
58-0
.258
-0.1
48-0
.448-0
.348
-0.2
48-0.
1
600
450
300
150
0
382818
COD & Interf * Casing Wall
COD & Interf * Steel Grade
Carbide OD,mm & Interference(%)
Casing Wall * Steel Grade
Casing Wall thickness,mm
182838
WallCasing
EN24H13W360
GradeSteel
Mea
n of
Ten
sile
str
ess
at s
teel
OD,M
Pa
Interaction Plot for Tensile stress at steel ODFitted Means
Fig.5. Interaction plot for tensile stress at steel OD
It can be observed from figure 4&5 that as the compressive stresses increases on the carbide the tensile stresses developed on the steel also increases. And by increasing carbide OD the compressive stresses on carbide decreases in turn increasing the tensile stresses on the steel casing, which is not feasible because the yield stress of steels is very less when compared to carbides. Hence stresses developed on the steel casing must be less than the yield stress of the steel material used. So selection of above factors is purely based on the application.
VI. SIMULATION TRAILS A. Simulation Trial No- 01 Interference=0.1% Carbide OD=48mm Steel grade=EN24 Wall Thickness=18mm
Fig. 6. Hoop stresses on die and casing
B. Simulation Trail No- 32 Interference=0.4% Carbide OD=48mm Steel grade=EN24 Wall Thickness=18mm Comparing the trial no-01 & trial no-32 shown in fig 6 & fig 7 it is observed that for each 0.1% increase in interference there is double the amount of stress generated. And small variation in interference can also make a huge difference in the stresses. Hence it is a major affecting factor to be considered.
International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 04, Volume 4 (April 2017) (SPECIAL ISSUE) www.ijirae.com
VII. SIMULATION PROCEDURE FOLLOWED FOR HOT FORGING USING SHRINK FITTED DIES. A .Die Stress at Room Temperature
Fig. 8. Hoop stresses on die and casing at room temperature
Fig. 9. Maximum principle stresses & pressure on die assembly at room temperature
The die insert used is tungsten carbide (WC-Co) and casing material is AISI 4340 steel. In the current simulation shown in figure 8 & 9 there is no internal pressure on the die insert. It is therefore a no-load simulation with only the stress distribution from shrink-fitting. After shrink fitting, the insert is under compressive hoop stress whereas the shrink rings are under tensile stress as expected. It should be noted that the die dimensions at the end of this process are different from the design dimensions due to elastic deflection at the shrink-fit interfaces. The simulation model with this elastic stress and deflection history is carried forward to the next analysis step [8]
B. DIE STRESS AT PRE-HEAT TEMPERATURE
Fig. 10. Hoop stresses on die and casing at preheat temperature
In order to determine the change in the pre-stress (and shrink-fit) with temperature a uniform preheat temperature was assigned to all die components with the stress and elastic deformation history from the room temperature shrink-fit simulation it is assumed that
International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 04, Volume 4 (April 2017) (SPECIAL ISSUE) www.ijirae.com
the dies are heated to a uniform preheat temperature of 150°C prior to the start of the forging process. The change in the pre-stress from thermal expansion of the die components is shown in Figure 10 & 11.
Fig. 11. Temperature & pressure on die assembly at preheat temperature
The compressive Hoop stress on the carbide die insert OD was found to drop by ≈ 24% in average due to change in thermal expansion of the die assembly. For a given die design this drop in pre-stress will increase with the preheat temperature and the difference in thermal expansion coefficients of the two die materials.
CONCLUSION
The stresses due to shrink fitting of carbide and steel mainly varies with interference and also with many other factors considered, and it can be seen that as the compressive stresses on carbide increases, Tensile stresses on steel also increases. But by increasing wall thickness tensile stress on steel can be decreased, hence developed stresses in any combination must be less than the yield stress of materials used. Presently the Optimization of shrink fitting for hot forging condition is under study. The hot extrusion die is simulated and analyzed up to the preheat temperature and future work to be done on next forging cycles.
ACKOWLEDGMENT
I would like to express my profound gratitude to R&DE material Science Department, Kennametal India Ltd. and NIE, Mysuru for the guidance and support in all my endeavors.
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