Failure Analysis of H13 Tool Steel in Aluminum Extrusion DiesII

1
Failure Analysis of H13 Tool Steel in Aluminum
Extrusion Dies-II

• Mesut Varlioglu, Graduate Student
• Taehyung Kim, Graduate Student
• Joe Benedyk, Professor
• Philip Nash, Professor
• Sheldon Mostovoy, Professor
• 11/14/03

2
AGENDA

• PART I- OVERVIEW
• The general information about our research goal.
• PART II-STEP LOADING TEST UPDATE
• Our findings on new step loading test to
  simulate the extrusion conditions in our
  laboratories.
• PART III- DETERMINING THE RETAINED AUSTENITE IN
  HEAT TREATED H13 TOOL STEELS
• A new method to determine the retained austenite
  in H13 tool steels that can be a detrimental
  effect on die steel.
• The essential parameters in Solid Metal
  Embrittlement.
• PART IV-LITERATURE REVIEW
• A discussion on a new article on Aluminum
  extrusion die failure.
• PART V-FUTURE WORK
• The material supplies for the electropolishing
  and Rising load test.

3
PART IOVERVIEW
4
PURPOSE

• The extrusion die is fractured in a short period
  of time and even far shorter than its desired
  service time.
• The parameters affecting the fracture of the die
  are our main objective in this research.

5
EXTRUSION PROCESS

• In schematical extrusion process, the preheated
  billet is forced to the die with the aid of the
  pressure and a compaction and better mechanical
  properties are obtained in single operation.

• Extrusion pressure 72,000 psi.
• Maximum extrusion temperature 1022 F (550
  C).

6
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.
• With its high fracture toughness and high
  strength, H13 is the most common die material in
  the industry.
• 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.
• Its relatively better mechanical properties with
  respect to 6061 which is the most common Al
  alloy, makes 7116 to find a usage in even
  aerospace applications where the high strength
  and low density is necessary.

7
OUR RESEARCH APPROACH

• The high Zn content in the extrusion material
  causes solid metal embrittlement phenemona in the
  die.
• Since the manufacturers of the extrusion dies
  are different, the chemical composition and
  alloying procedures and most importantly heat
  treatment processes can vary. So, it is also best
  to learn the chemical compositions and parameters
  that the manufacturing companies are using must
  be known in order to learn the reason of the
  fracture of the die.
• Because the manufacturers are using the
  "slightly" different parameters in the process up
  to final product, we conclude that Zn, the major
  element in the extrusion material, causes the
  Solid Metal Embrittlement in the die.

8
PART II STEP LOADING TEST UPDATE
9
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.
10
Specimen Shape
Specimen cross section was calculated for 200 ksi
tensile strength and 12 kips max. load for the
Instron Machine.
Specimen after machined.
11
Heat Treatment Schedule
To eliminate the decarburization layer of the
surface, it was planned to heat treat the sample
in vacuum furnace. First, we have to find out
what is the heat treatment procedure to obtain 42
HRC which is the hardness value of the extrusion
die. Also, cooling will be a problem because most
steel is air cooled and when we air cool the
sample, the surface will corrode. In order to
eliminate this problem, it was planned to do heat
treatment in a tube and to take the tube out
after the heat treatment stage.
12
Hardness Profile for 1850 F
13
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.
14
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)
15
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.
• Guidelines for MIE in base metal and embrittler
• Not to form intermetallic compounds.
• Not to solute each other substantially.
• Have similar electronegativity values.
• Source A.P. Druschitz, P. Gordon, Solid
  Metal-Induced Embrittlement of Metals, IIT.

16
PART III DETERMINING THE RETAINED AUSTENITE IN
DIE STEEL
17
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 1 hour,
  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.
• X-Ray diffraction requires thin sample and need
  more time so, a metallographic method is
  successful for retained austenite calculation.

18
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.
19
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.
20
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.
21
First Attempt
20 ?m
(a) (b)
H13 sample that austenitized at 1950 F in 1 hour,
then air cooled. 1000x. a) Etched with Nital. b)
Electropolished and copper colored.
22
PART IV ARTICLE REVIEW
23
Failure Analysis of Al Extrusion Dies

• In this article, 17 different die profiles and
  616 die failures in commercial extrusion of 6063
  were studied.
• Mainly the article gives a statistical approach
  for the die failure.

24
Extrusion Die Picture
25
The results
26
The Review

• The article has a statistical value.
• It doesnt discuss the parameters that can cause
  the failure.
• Die material was not discussed since 616 die
  failures were studied.
• The extrusion material, 6063, is different (our
  die material is 7116).

27
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.
• Finding the optimum heat treatment procedure to
  obtain 42 HRC is essential.

28
FUTURE WORK

• To continue the SEM analysis on the die not
  exposed to caustic.
• To heat treat the Rising Load samples with
  vacuum atmosphere.
• To continue the metallographic method for
  revealing the retained austenite and compare the
  results with other dies as well as Rising Load
  Test specimens.
• Study austenitization and tempering conditions
  to obtain 42 HRC.
• Study the electronegativity, solute solubility
  and intermetallic compouds of die material and
  Zinc.
• Continue investigating fracture surfaces in
  different dies.