Failure Analysis of H13 Tool Steel in Aluminum Extrusion Dies

1
Failure Analysis of H13 Tool Steel in Aluminum
Extrusion Dies

• Mesut Varlioglu, Taehyung Kim
• Joseph Benedyk, Philip Nash, Sheldon Mostovoy
• Thermal Processing Technology Center
• 12/02/03

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OUTLINE

• I- Overview
• General information about research goal.
• II- Fracture Surface Analysis
• Our methodology on the evaluation of die fracture
  behavior and parameters affecting the short-time
  failure of the extrusion die.
• III- Molten Zn Immersion Test
• A simulation on Liquid Metal embrittlement
  phenomena of the die.
• IV- Rising Load Test
• Our findings on new step loading test to simulate
  the extrusion conditions.
• V- Metallographic Technique for Retained
  Austenite in H-13
• Details and some applications.
• IV- Future Plan
• Our future direction.

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PURPOSE
H13
FAILURE
H-13
Heat550 C.
7XXX Al
Why the H13 extrusion die with Al-Zn-Mg alloy
fractures in a short period of time?
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MATERIALS IN EXTRUSION PROCESS

• DIE MATERIAL
• H13 (wt) 0.32-0.45C, 0.80-1.25Si, 0.20-0.60Mn,
  4.75-5.50Cr, 1.10-1.75Mo, 0.80-1.20V.
• Mostly carbide formers in the chemical
  composition.
• Common die material with its high fracture
  toughness and high strength.
• EXTRUSION MATERIAL
• 7116 (wt) 0.15Si, 0.30Fe, 0.50-1.1Cu, 0.05Mn,
  0.8-1.4Mg, 4.2-5.2Zn, 0.05Ti, 0.05Ga.
• Used for high strength applications.

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II FRACTURE SURFACE ANALYSIS
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EXTRUSION DIES AFTER FRACTURE
Fracture Initiation
The back part of the die.
The front part of the die.
Fractured die as received. The manufacturer
Belgium MATEC. The manufacturing date 09/2002
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CUTTING DIRECTIONS
The die block.
Another look to the fractured surface.
The fractured surface that came out.
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Fracture Surface
Bridge
Fractured surface after cleaning the
surface. Note the three different fractured
regions. Fractured with a fatigue crack
propagation behavior.
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Three Layers in the Fracture Surface
Crack Initiation Layer (CIL) Al Exposed
Layer (ALL)
17 mm
6 mm
20 mm
SEM Analysis Regions
Fractured surface in a closer look.

Crack Arrest Layer (CAL)
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1- Crack Initiation Layer (CIL)
Crack initiation region which is the smallest
cross section in the die.
Fractured surface in a closer look.
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SEM Analysis for CIL
Crack initiation place
The fractured surface in a more closer look. The
chemical analysis shows that the Zn content in
the fracture initiation layer (CIL) is higher
than the other regions.
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2- Al Exposed Layer (ALL)
Al exposed layer is mainly characterized as high
Aluminum content (app. 60). Zinc content is
around 4 in this layer. High Aluminum content
can be caused by the crack opening in each cycles
in extrusion process and then Al can fill in this
layer.
Fractured surface in a closer look.
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SEM Analysis for ALL
High Zn content (around 20)
Composition is like base material (H13).
Al rich (90) region. Zn is around 4.
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SEM Analysis for CAL (Crack Arrest Layer)
Al rich regions
Composition is like base material (H13) with low
content of Al.
Composition is like H13 tool steel.
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Hardness Test of the Fracture Surface

• After the fractured part was removed from the
  die, a cutting pattern difference was observed
  along the die profile.
• It is observed if there is a case hardening
  effect in the bridge of the die.

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11
10
13
9
8
7 6 5 4 3 2
1
14
20
19
15
18
17
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After the hardness, it is concluded there is no
case hardening affect in the surface of the die.
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Fracture Surface in the first die
Cutting Direction
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SEM Analysis for the die fracture surface
Crack initiation region which is the smallest
cross section in the die.
1
5
2
6
7
3
4
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Fracture Surface Analysis for the die exposed to
aluminum.
The previous crack surface investigation was done
with the dies exposed to caustic. A slice with 10
mm thick from the center of the die was cut.
2
1
Die parts after cutting operation.
The Die piece. Red circles show the area observed
in SEM.
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SEM Pictures
1
2
3
7 6 5 4 3 1 2 8 9
Area 1 Area 2
The chemical distributions of Zn and Al in the
interface of fracture and interaction area.
With poor precision.
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Surface Analysis of the extrusion material
2
1
Inside of the extrusion material.
Material surface.
Chemical compositions of two points. With poor
precision.
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Heat Treatment Process Simulation
An austenitizaton at 1850 F, air cooling, and
double/triple tempering at 1150 F for H13 tool
steel is the acceptable heat treatment procedure
at most die shops. We analyzed the heat
treatment procedure to vary the austenitization
temperature.
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Experimental Procedure
Revised for 1750 F and 1950 F
Revised for different tempering times
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Hardness Profile for 1850 F
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Microstructures - 400x
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Microstructures - 1000x
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Microstructures in different austenitization
temperatures
(a)
(b)
H13 samples that were austenitized at different
temperatures. Nital. 1000x. a) 1750 F, b)1850 F,
c) 1950 F. Note the bigger grains in 1950 F.
(c)
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III MOLTEN Zn IMMERSION TEST
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Microstructures with a new etchant
Micrograph for H13 austenitized at 1950 F, 1
hour, air cooled and 1100 F for 4 hours, air
cooled. 1000x.
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Zn Immersion Test

• The samples made of H13 tool steel and
  austenitized at 1850 F for 1 hour, then air
  cooled are immersed in molten Zinc in different
  times to observe if the Zinc diffuses the die
  material.
• In order to reduce the oxidation of molten Zinc,
  charcoal is sprinkled to the molten Zinc surface.
• After the Zn immersion test, the samples are cut
  in transverse direction, mounted and etched with
  Nital.
• An SEM analysis has also been conducted and the
  chemical distribution on the cross section has
  been examined.

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Chemical Distribution of Zn
Zn distribution for the sample that immersed in
molten Zn in 25 hours
1 2 3 4 5 6 7
8
Surface
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Step Loading Test Procedure

• The tensile test specimen that has a cylinder
  gap inside is filled with 7XXX series Al alloy
  and heated in 550 C.
• Then, the inside part is closed with a screw.
• In the high temperature tensile test machine,
  the specimen is heated at around 550 C and
  started to load at P/8 in each cycles in 8 hour
  period.
• Then the data from the specimen filled with
  other substances including air is compared. Also,
  the fracture surface is compared with the real
  fracture surface of the bridge of the die.

Source ASTM F 1624-95.
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Specimen Shape
Specimen cross section was calculated for 200 ksi
tensile strength and 12 kips max. load for the
Instron Machine.
Specimen after machined.
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Heat Treatment and Test Conditions

• Heat Treatment
• Atmosphere Nitrogen.
• Austenitization Temperature 1850 F, 1 hour.
• Hardness 52 HRC.
• Tempering Temperature 1100 F, 6 hrs.
• Hardness 49 HRC.
• Rising Load Test
• Loading Rate 5 lbs/s.
• Test Temperature 521 C.
• Load in each step 1.5 kips (8 steps).

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First Test Result
Stress 142 ksi No fracture.
Load vs. Time
Load vs. Displacement
Test material H13 filled with 7116 Al alloy.
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Second Test
Stress 180 ksi Fracture
after 9.8 hours.
Load vs. Time
Mechanical properties of H13 at 550 C
Source ASM Specialty Handbook, Tool Materials,
1995, p.140-141.
Same Sample.
Load vs. Displacement
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Solid Metal Embrittlement (SME) vs. Rising Load
Test
SME is the phenomenon that when a normally
ductile metal is in intimate contact with certain
low melting point metals and simultaneously under
tensile stress, cracking at abnormally low
stresses occur. This embrittlement can also occur
at temperatures well below the embrittler melting
point. Metal Embrittlement has 3 stages 1.
Incubation period If the embrittler is removed
before a crack nucleates, mechanical properties
of base metal will be same. 2. Embrittler-controll
ed crack propagation Rate of crack propagation
being fixed by rate of transport of embrittler to
the tip of the crack. 3. Sudden Failure The
stress at the crack tip is sufficient for normal
ductile crack growth. Embrittler has no effect in
this stage. Source A.P. Druschitz, P. Gordon,
Solid Metal-Induced Embrittlement of Metals, IIT.
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Correlation with Fracture Surface of the die
Crack Initiation Layer (CIL) Al Exposed
Layer (ALL)
17 mm
6 mm
20 mm
SEM Analysis Regions
Fractured surface in a closer look.
Crack Arrest Layer (CAL)
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The characteristics of SME

• Only occurs with base metal and lower melting
  point embrittler.
• Intimate (atomic) contact must occur between two
  metal.
• Tensile stress is required, internally or
  externally, simultaneously to the contact area.
• Threshold stress must exist.
• A discontinuity in crack initiation time occurs
  at the embrittler melting point.
• Source A.P. Druschitz, P. Gordon, Solid
  Metal-Induced Embrittlement of Metals, IIT.

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IV DETERMINING THE RETAINED AUSTENITE IN DIE
STEEL
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Purpose

• The optimum heat treatment procedure of H13 tool
  steel
• Austenitization at 1850 F for 1 hour, air
  cooling.
• 3 consecutive tempering at 1150 F for 2 hours,
  air cooling.
• This heat treatment procedure has high cost for
  die machine shops and they may do single
  tempering (increased tempering time) to obtain 42
  HRC which is the optimum hardness value.
• The retained austenite will be in the
  composition and it is undesirable for the service
  life of the tool steel.
• The penetration layer in X-Ray diffraction is
  thin and sample preparation need time so, a
  metallographic method can help to assess XRD data
  for retained austenite determination.

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Metallograhic Method for Retained Austenite
Calculations
The method can be summarized with 4 stages 1-
Mechanical Polishing 2- Electrolytic Polishing 3-
Copper Deposition 4- Copper Coloring Source
E.J. Klimek, A Metallograhic Method for Measuring
of Retained Austenite, 1995.
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Mechanical and Electrolytic Polishing

• 1- In mechanical polishing, the sample is
  polished until 0. 05 µm alumina.
• 2- In electrolytic polishing
• Equipment consists of
• Direct power source (0-10 V, 0-5 A)
• Electrolyte in a 250 ml beaker, 10 gr CrO3, 100
  ml distilled water.
• Conditions 4 V, 1.6 A/in2, 15 to 30 s.

Source Vander Voort, ASM Desk Editions, 2001.
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Copper Deposition and Coloring
3- Copper Deposition Solution of 955 ml H2O,
14.2 gr CuSO4 and 7.4 ml H2SO4 is applied to the
sample surface by either agitated immersion or
daubing with a saturated cotton swab in 5 to 10
seconds. The copper deposition can be seen by
green light filter. 4- Copper Coloring Solution
of 1 gr Na2S, 100 ml distilled water, 1 ml HNO3
in ph of 5 is applied to the sample surface by
cotton in 5 to 20 seconds. Deposited copper
color varies with time, ranging brown to blue to
black. This process was tried for 8615 steel
ring gears and 5120 steel. We will adjust this
method for the H13 tool steel.
Source E.J. Klimek, A Metallograhic Method for
Measuring of Retained Austenite, 1995.
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Results
1750 ºF 1850 ºF
1950 ºF
20 ?m
0 h
0 h
0 h
(a) (b)
8 h
8 h
8 h
H13 samples that austenitized at different
temperatures and tempered at 1100 ºF at different
times. 1000x.
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CONCLUSIONS

• The Retained Austenite can be a parameter for
  the extrusion die failure.
• Zn as part of Al alloys may cause embrittlement
  in hot extrusion.
• The Rising Load test can be a good model for
  studying the hot extrusion applications as well
  as Zinc effect on the die.

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FUTURE WORK

• To continue the SEM analysis on the die not
  exposed to caustic.
• To continue the metallographic method for
  revealing the retained austenite and compare the
  results with other dies as well as Rising Load
  Test specimens.
• Duplicate the rising load test results by using
  different filling material, testing temperature,
  loading rate.
• Continue investigating fracture surfaces in
  different dies.