EFFECT OF DESIGN FACTORS ON THERMAL FATIGUE CRACKING OF DIE CASTING DIES John F. Wallace David...
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Transcript of EFFECT OF DESIGN FACTORS ON THERMAL FATIGUE CRACKING OF DIE CASTING DIES John F. Wallace David...
EFFECT OF DESIGN FACTORS ON THERMAL FATIGUE CRACKING OF DIE CASTING DIES
John F. WallaceDavid Schwam
Sebastian BirceanuCase Western Reserve University
NADCA Die Materials Committee Meeting
Rosemont, March 6, 2002
EFFECT OF MAXIMUM TEMPERATURE ON THERMAL FATIGUE DAMAGE
OBJECTIVE
• Determine the effect of the maximum temperature on the thermal fatigue cracking at the corners of the 2x2x7” H13 specimen.
APPROACH
(1) Vary the immersion time (5, 7, 9, 12 sec.), while the overall cycle time remains the same (36 sec).
(2) Vary the cooling line diameter 1.5”, 1.6”, [1.7”], 1.8”), without changing the cycle time (9 sec. immersion time, 36 sec overall cycle time.)
The Maximum Temperature Cycle for 5 sec Immersion
0
200
400
600
800
1000
1200
0 6 12 18 24 30 36
Time [sec]
Te
mp
era
ture
[F
]
max 1008 F
The Maximum Temperature Cycle for 7 sec Immersion
0
200
400
600
800
1000
1200
0 6 12 18 24 30 36
Time [sec]
Te
mp
era
ture
[F
]
max 1130 F
Temperature Variation at the Corner of 2x2x7 H13 Specimen
The Maximum Temperature Cycle for 9 and 12 sec Immersion Time
0
200
400
600
800
1000
1200
1400
0 6 12 18 24 30Time [sec]
Tem
per
atu
re [
F]
12 sec immersion time
9 sec immersion time
Maximum Temperature Cycle for 9 and 12 sec Immersion - Close-Up
0
200
400
600
800
1000
1200
1400
3 5 7 9 11 13 15 17Time [sec]
Tem
per
atu
re [
F]
12 sec immersion
9 sec immersion
max 1212 F max 1228 F
Total Crack Area vs. Maximum Temperature at the Corner of 2x2x7 H13 Specimen For Different Immersion Times
1008 F
1228 F
1212 F
1130 F
0
20
40
60
80
100
120
140
160
180
1000 1050 1100 1150 1200 1250
Temperature [F]
To
tal C
rack
Are
a [x
10
6
m2]
Corner, 15000 cycles
5 sec 7 sec
9 sec
12 sec
CR
TIC
AL
TE
MP
ER
AT
UR
E
TOTAL CRACK AREA OF H13-45HRC5, 7, 9 and 12 sec immersion time
0 0.37 1.970.09 0.875.9
0.19 0.19
39.69
167.72
17.53
108.56
0
20
40
60
80
100
120
140
160
180
5000 10000 15000
Thermal Cycles
To
tal C
rac
k A
rea
(x
106 m
2)
H13-45 HRC/5 sec immersion time
H13-45 HRC/7 sec immersion time
H13-45 HRC/9 sec immersion time
H13-45 HRC/12 sec immersion time
2"X2"X7", WC7
AVERAGE MAX CRACK LENGTH OF H13-45HRC5, 7, 9 and 12 sec immersion time
0
2
4
6
8
10
12
14
16
18
20
5000 10000 15000
Thermal Cycles
Avera
ge M
ax C
rack L
en
gth
(x
100
m) H13-45HRC, 5 sec immersion time
H13-45HRC, 7 sec immersion time
H13-45HRC, 9 sec immesion time
H13-45HRC, 12 sec immersion time
2"X2"X7", WC7
7 9 12
12
12
9
9
77
5
5
7 in(178 mm)
3.5 in(89 mm)
• The hardness is measured at the center of the specimen,beginning at 0.01 in (0.254 mm) from the edge.• The next testing steps: 0.02in (0.508 mm), 0.04in (1.016 mm), 0.06in (1.524mm), 0.08in (2.032mm), 0.1 (2.54mm), 0.15in , 0.2in then in 0.1in increments until no further variation of hardness occurrs
MICRO-HARDNESS MEASUREMENT
Softening of the 2x2x7 H13 Specimen
15
18
21
24
27
30
33
36
39
42
45
2.54 5.08 7.62 10.16 12.7 15.24 17.78 20.32 22.86 25.4
Distance from the Corner [x100 m]
Ha
rdn
es
s H
RC
H13/45 HRC, 5 sec immersion
H13/45 HRC, 7 sec immersion
H13/45 HRC, 9 sec immersion
H13/45 HRC, 12 sec immersion
The hardness testing begins at the corner
5 sec
7 sec
9 sec
12 sec
Note: Longer immersion times cause more severe softening
Hardness Loss vs. Immersion Time for Different Distances from the Corner
0
5
10
15
20
25
3 4 5 6 7 8 9 10 11 12 13
Immersion Time [sec]
Har
dnes
s Lo
ss [H
RC
]
0.02"
0.06"
5 sec
7 sec
9 sec
12 sec
The loss in hardness is most severe at the corner and becomes less severe further away
Distribution of the Carbides in the Thermal Fatigue Specimen
A mechanism of softening at the corners is carbide coarsening
MECHANISM OF THERMAL FATIGUE CRACK NUCLEATION AND PROPAGATION
• Most new H13 die have sufficient strength to resist immediate formation of cracks.
• After being exposed to thermal fatigue cycling, the hot areas of the die will soften, thereby losing strength. When the fatigue strength of the steel drops below the operating stresses cracks will form and propagate.
• Crack propagation is gradual and controlled by the gradual softening that progresses with time deeper into the die.
Note: If the operating thermal stresses combined with stressconcentration factors exceed the fatigue strength of the steel, fatigue cracks can propagate even w/o softening.
TOTAL CRACK AREA of 2X2X7 H13 SpecimenDifferent Cooling Line Diameters
0.19
17.53
108.56
0.18
13.96
0.1813.81
35
79.44
0
20
40
60
80
100
120
140
160
180
5000 10000 15000
Number of Cycles
To
tal C
rack
Are
a (x
10
6
m2)
1.5" cooling line1.6" cooling line1.8" cooling line 2"X2"X7", WC7
1.5”
1.6”
1.8”
AVERAGE MAXIMUM CRACK LENGTH of 2x2x7 H13 Specimen - Different Cooling Line Diameters
0.85
5.25
12.5
0.75
5.25
12.25
0.75
5
8.25
0
2
4
6
8
10
12
14
16
18
5000 10000 15000
Number of Cycles
Av
era
ge
Ma
x C
rac
k L
en
gth
(x1
00
m
) 1.5" cooling line1.6" cooling line1.8" cooling line
2"X2"X7", WC7
1.5”
1.6”
1.8”
Hardness Variation for Different Cooling Line Diameters(15000 cycles)
20
25
30
35
40
45
50
55
2.54 7.62 12.7 17.78 22.86
Distance from the Corner [x100 m]
Ha
rdn
es
s H
RC
1.5" cooling line1.6" cooling line1.8" cooling line1.5”
1.6”
1.8”
0
2
4
6
8
10
12
14
900 950 1000 1050 1100
Corner Temperature [F]
Ave
rage
Max
imum
Cra
ck L
engt
h [x
100
m
]
1.8"
1.7"
1.6" 1.5"
15, 000 cycles
The Effect of Cooling Line Diameter on Average Maximum Crack Length
The larger the cooling line the more heat it removes, thus lowering the temperature, reducing softening and cracking
Str
ess
[psi
]
Time [sec]
Experimental Data for Stress in the 1.5 “Cooling Line of 2x2x7 H13 Specimen
0
20000
40000
60000
80000
100000
120000
36 72 108 144 180 216 252
Details of Through Cracks on Sides of 2x2x7” Specimen(annealed; off-center cooling line, 5000 cycles)
CONCLUSIONS
• Below a certain temperature threshold the thermal fatigue damage is minimal; this observation applies to the ground 2”x2”x7” H13 specimen tested to 15,000 cycles, in the absence of high stress concentrators.
• The thermal fatigue damage is mainly determined by the temperature-time cycle, the thermal stresses and the softening of the specimen.
• A longer dwell time at high temperature is more damaging thana short one. This is because of the accelerated softening effect athigh temperature.
CONCLUSIONS (continued)
• Long dwell times at high temperature simulate die casting of large components, where the die surface is subjected to elevated temperature for longer periods of time.
• The experimental results demonstrate less thermal fatigue damage when the cooling line is closer to the surface and lowers the temperature.
• However, bringing the cooling lines closer to the surface may cause high hoop stresses in the cooling line and at the surface. This may increase the danger of gross cracking.
METHODS OF KEEPING “HOT SPOTS” IN DIES BELOWCRITICAL TEMPERATURE
1. Longer cycle time that allows die to cool - slows production.
2. More insulating die lubricants - slows production.
3. More water spray - danger of thermal shock.
4. Die materials with better heat diffusivity.
5. Larger cooling lines drilled closer to hot spots - accessibility.
6. Optimized use of Baffles and Bubblers.
EVALUATION OF BAFFLES AND BUBBLERS
OBJECTIVE
Compare the efficiency of commercially available baffles and bubblers in removing heat from “hot spots”.
METHOD
• Use standard size OD/ Length H13 specimen inside furnace.
• Vary internal cooling line diameter and water flow rate.
• Use inter- changeable baffles and bubblers.
• Compare outlet water temperature and specimen temperature for constant inlet temperature.
Flow
Furnace
Water Inlet
Water Outlet
Meter for Flow Rate and Temperature
Specimen
Set-up for Evaluation of Baffles and Bubblers
Data Acquisition
Hole for Thermocouple
Baffle
Water flow
Water-inWater-out
Schematic of Baffle-cooled specimen
HOT SPOT!
"Hot Spot" Temperature for 0.2" ID Bubbler vs. 0.3" ID Bubbler
500
550
600
650
700
750
800
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Flow Rate [gal/min]
Tem
per
atu
re [
F]
0.3" ID
0 0.2" ID
Furnace at 1800oF
INTERIM CONCLUSIONS
• For identical water flow rates, smaller diameter bubblers generate a higher flow velocity and are more efficient in cooling a localized hot spot.
• Baffles and bubblers can be used to reduce the local temperature of “hot spots” in the die below critical temperatures that accelerate soldering.
• Surgical needle-size bubblers are commercially available for cooling hard-to-access hot spots and thin sections.
• Further experiments are planned to compare the cooling efficiency of different designs of baffles and bubblers.