Improved Life of Die-Casting Dies [H-13 Steel] by Heat Treatment

255 Pages·1998·10.245 MB·English

DOE/ID/13320-3
Improved Life of Die Casting Dies of H13 Steel by Attaining
Improved Mechanical Properties and Distortion Control
During Heat Treatment
Final Report
J. F. Wallace
D. Schwam
October 1998
Work Performed Under Contract No. DE-FC07-941D13320
i
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For
U.S. Department of Energy
Assistant Secretary for
Energy Efi?ciency and Renewable Energy
Washington, DC
By
Case Western Reserve University
Cleveland, OH
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DOEAD113320-3
IMPROVED LIFE OF DIE CASTING DIES OF H13 STEEL BY ATTAINING
IMPROVED MECHANICAL PROPERTIES AND DISTORTION CONTROL
DURING HEAT TREATMENT
FINAL REPORT
J. F. Wallace
D. Schwam
October 1998
Work Performed Under Contract No. DE-FC07-941D13320
Prepared for the
U.S. Department ofEnergy
Assistant Secretary for
Energy Efficiency and Renewable Energy
Washington, DC
Prepared by
Case Western Reserve University
Cleveland, OH
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DISCLAIMER
This report was.prepared as an account of work sponsored
byan agency of the United States Government. Neither
the United States Government nor any agency thereof, nor
any of their employees, make any warranty, express or
implied, or assumes any legal liability or responsibility for
the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or
represents that its use would not infringe privately owned
rights. Reference herein to any specific commercial
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manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by
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views and opinions of authors expressed herein do not
necessarily state or reflect those of the United States
Government or any agency thereof.
DISCLAIMER
Portions of this document may be illegible
in electronic image products. Images are
produced from the best available original
document.
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ABSTRACT
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Optimum heat treatment ofdies,quenchinginparticular,arecriticalinensuringsatisfwtory perfornxuMxh
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service.Rapid coohg ME i.nc- thethe~ fatigue or ~t chec~g resistance of the skeL However, very
tit cooIing rates can also lead to distortion and lower fiactum toughness values, increasing the danger of
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catastrophic &Wure.
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TIE ukirnate goal of this pro&t is to increasedielife byusingf=t enoughquenchingrates(morethan
I 301%ninaveragecoofig ratefrom1750~ to550Y - 1/2”belowworhg s-) to obtaingood toughness and
M.igueresistanceinl%mium GradeH-13 steeldies. The maintash ofthe pro-t were
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CompiIedatabase onphysicalandmechanicalpropertiw ofH-13.
1 Conduct gasquenchingexperimentsto determinecooling rates ofdiesindiiferentvacuumfurnaces.
Measwe the asquenched distortionofdiesandtheresidualstresses.
I Generate Ftite Element Analysis models to predict cooIing rates, distortion and r&dual stms of gas
quencheddies.
1 Establishrulesandcreate PC-basedexpert systemfor prediction of coolingrates, distortion and residualstress
invacuum/gasquenchedH-13 dies.
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h iterative approach of computermodetig v*M by expe~en~ -Urements was Mm =.
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illustratedinfigwe 1. Inthefirstcycleasimpleshape,XWtangularblock wasusedfor the computer model andthe
experiments. Subsequently,modelingofamore intricatedieshapewas undertaken.
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CoolingcurvesduringgasquenchingofH-13 blocks anddieshapeshavebeenmeasumd under avarietyof “
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gaspresmues,i.e. 2 bar, 5 bar, 7.5, 10barnitrogen,4 bar argon, 20 barnitrogen+ helium and 01 Dimensional
changes caused bythe above gas quenching processes have been determined by auwrate mapping of allsurfaces
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with Coordinate Measuring Machines befo~ and after the quench. Residual stresses were determined by the
ASTM E837 holed~g strain gage method. To facilitate the computer modeling worlG a comprehensive
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database of H-13 mechanical and physiwdproperties has been compiled. Ftite Element Analysis of the heat
tm.atedshapeshasbeenconductedusingtheTRASTIABAQUS codes.
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In
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The pro@t has successfully completed all the experiment go*. De~~ distortion mapping of kge H13
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blocks and dies was conducti with a Cm machine. Concomitantly, the coo~g CUWeSat d~erent
locations in the bloclddie were measurd. This procedure was rep=ti for different quenching gas
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pressures, utilizing stateof the art vacuum heat treating furna= A quantitative correlation WaSestablished
for the block shaped pw@between the cooling rate from austenitig temperature and the cooling rate.
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Essentially, in the expefienti window of lodegrees Fhnin to 150degr= F/* measured in these large
blocks and dies, the higher cookg rates yield more distortion. The main sources of distortion are the plastic
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deformation occurring early on in the quench and the volume changes induced by the phase transformation.
As the outside of the part COOISand contracts, it applies kge str=ses on the inside that is stillhot, hence
causing plastic deformation. Ultimately, the distortion isdetermined by how thick and strong the colder
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outside ‘shell”is. If the “sheu”k strong enough to resist the intem~ strws= appfied bythe hot core, the part
will not distort excessively. kdustfi heat treating practice includes means to control excessive distortion
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by interrupting the quench before the onset of the martensitic transformation. This interruption slows down
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the cooling rate around 750 degr=s F-1OOOdegrees F. This stabb the internal stresses and allows the
surface and core temperatures to equalize. The nitrogen gas quench k normally continued after these
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temperatures are within XW~. The interrupted quench combines the benefits of a fast cooling rate inthe
critical range with an acceptable distortion leveL The controlled experiments conducted by this project
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demonstrate a significant decrease indistortion of 30-40% facilitated by the interrupted quench procedure.
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The computer modeling part of the project was successful in handling the temperature distribution in
I the part during the quench. Good fit of modeled vs. measured quenched rates has been demonstrated for
simple die shapes. The models also predict well the phase transformation products resulting from the quench.
I These models assume no distortion during heating to the austenitizing temperature, which isthe case with
slow heating. The modek account for latent heat, thermal and phase transformation strains as wellasplastic
1 deformation.
I There is a good fit between the predicted and measured distortion contours. However, the magnitude
of the predicted distortion and residual stresses does not match well the measured values. Further fmetuning
I of the model isrequired before it can be used to predict distortion and residual stress in a quantitative
manner. This last step isa prerequisite to generating rules for a reliable expert system.
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LIST OF CONTENTS
Literature review
Prediction of cooling rates
Experimental cooling rates
Die materials properties data base
4.1 General
4.1.1 Commercial Designation
4.1.2 Alternate Designations
4.1.3 Specifications
4.1.4 Composition
4.1.5 Heat Treatment
4.1.6 Hardness
4.1.7 Forms and Conditions Available
4.1.8 Melting and Casting Practice
4.1.9 Special Considerations
4.2. Physical Properties
4.3. Mechanical Properties
4.3.1 Specitled Mechanical Properties
4.3.2 Mechanical Properties at Room Temperature
4.3.3 Mechanical Properties at Various Temperatures
4.3.4 Creep and Creep Rupture Properties
4.3.5 Fatigue Properties
4.3.6 Elastic Properties
5. Finite Element Analysis Models of an H-13 Block
6. Experimental Verillcation of Thermal, Residual Stress and Distortion Models of the Block
7. Experimental Results of Vacuum/Gas quenching of H-13 Dies
8. Bibliography
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I. INTRODUCTION
This report outlhws work done under CWRU Subproject OPE-R&D-95-212 “Improved
Life of Die Castings of H-13 by Attaining Improved Mechanical Properties and Distortion
Control During Heat Treatment”, sponsored by the Department of Energy, under the Metal
Casting Competitiveness Research Program. It is part of the a larger project on “Research,
Development and Demonstration of Die Casting Research to Increase the Competitiveness of the
U.S. Foundry Industry”. The larger project was submitted by Ohio State University, that
subsequently coordinated and monito~d the subprojects.
Finite Element Analysis and computer simulation are revolutionizing design and
manufacturing processes. Application of these techniques results in significant time and money
savings. Prototyping and preliminary designs of products that used to take months, and involve
expensive iterative trial and error runs can be shortened or eliminated. A critical aspect of using
these methods in die fabrication is availability of reliable materkd data. In addition, the computer
predictions need to be carefully validated on select models, before using them on a wider scale.
The report includes a compilation of material properties for H-13 and related steels. A
format similar to the Aerospace Structural Metals Handbook was adopted for this compilation
to facilitate comparison with H-11, covered by this source. A bibliography of publications on
these steels is also included in the repo* These am provided in print as well as Procite fdes.
Control of distortion during heat treating of die steels is acritical aspect of die fabrication,
with major technical and economic implications. Currently, dies can be brought to final
dimensions within the tight required tolerances only after final heat treatment. This behooves
die makers to leave generous feedstock to be removed in the hard, tough, hardened condition by
expensive grinding or to utilize Elecro-Discharge Machining (EDM) methods, followed by more
heat treatment. Ideally, die makers would prefer to machine dies to final dimensions before heat
treatment. In the present generation of die steels, some heat mating distortion is unavoidable
if the best mechanical properties are to be attained. It follows that the only practical way to
minimize post-heat treatment machining would be to predict distortion and offset it in advance
by allowing the right amount of material at the right locations.
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Many attempts have &n made over the years to predict and control quenching distortion.
The few successful cases were limited to very speciilc part geometries and processing conditions.
The multitude of parameters that affect distortion makes this task ex~mely difficult to
accomplish. A fascinating example is the effect of quenchant flow di~ction on the distortion.
Everything else being identical, the distortion measured off round cylinders under such cooling
conditions shows a remarkable dependency on the direction of the flow.
Two recent developments are turning prediction of heat treating die distortion by die
makers into a mom feasible task:
(a) Vacuum/pmsure gas quenching of dies. As this method becomes more common, there
is a better chance to achieve improved, reproducible control over the quenching process.
(b) Availability of lower cost Finite Element Analysis computer software, and the necessary
computer hardware to run it- At the time of this writing, the availability of FEA codes
for heat mating distortion is very limited, and requires ~latively expansive workstations.
However, many FEA softwrue houses are beginning to offer PC versions of their codes
and the Pentium PC computers are capable of running such applications.
1. Literature Review “
The distortion that accompanies heat treatment has been a problem to the heat treaters
over the years. Factors affecting distortion and the different means to control it has been the
subject of various fikmture. These factors include the structure and composition of the steel, the
properties, residual stress, cross-s@ion, and surface condition [1Z,3,4]. Another important factor
is the heat treatment process itsew, the fbrnace characteristics, the quenching operation
[1,2,3,5,6,7] and even the racking operation [3,7] contibute to the distortion.
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“- Based on “Prediction of Heat-up and Distortion in the Heat Treatment of Steels”, M.SC.Thesis
by Melmx Ordillas, Case Western Reserve University, May 1994. Adviser, Prof. J.F. Wallace.