Advanced High-Strength Steel Repairability Study: Phase I Final ...

General Motors-AISI AAC Advanced High Strength Steel Repairability Study
Phase I Final Report
Introduction
The introduction of Advanced High Strength Steels (AHSS) to light vehicle body structure
applications poses a significant challenge to organizations involved in the repair of vehicle
structures. AHSS are typically produced by nontraditional thermal cycles and contain
microstructural components whose mechanical properties can be altered by exposure to elevated
temperatures. This temperature sensitivity could alter the mechanical behavior of AHSS after
exposure to elevated temperatures during repair welding or flame straightening and could
seriously affect the structural performance of AHSS components after repair. This study,
requested by General Motors' Collision Repair Tech. Center, examined the mechanical behavior
of two AHSS products, a 600 MPa tensile strength (TS) dual phase steel and a 1300 MPa TS
Martensitic steel, after exposure to typical repair arc welding and flame straightening
temperature cycles and developed recommended practices for repairing components made of
these materials.
This study is the first phase of and anticipated multi-phase effort to characterize AHSS and
develop appropriate repair procedures for their various grades. Phase I objectives were to
develop an appropriate method for characterizing the mechanical behavior of AHSS upon
exposure to typical repair procedures and to develop recommendations for heat application
during the repair of parts made from the dual phase and martensitic grades mentioned above.
These AHSS grades are being specified for near-term General Motors vehicle platform
components and require immediate repair procedure development. Subsequent phases of this
activity will develop repair procedures for other AHSS grades as prioritized by General Motors
or other auto OEM AHSS component introductions.
Phase I was conducted by a team composed of General Motors, AISI, and AISI Automotive
Applications Committee steel company representatives. Team members included:
Member
Brian Dotterer
Jim Fekete
David Anderson
David Hoydick
Steve Kelley
Nassos Lazaridis
Blake Zuidema
Affiliation
General Motors Corporation
General Motors Corporation
AISI (AISI Coordinator)
United States Steel Corporation
International Steel Group
Ispat Inland, Inc.
International Steel Group
The results, conclusions, and recommendations contained herein are the consensus views of the
team members.

Procedure
Materials
Conventional and AHSS steel grades selected for this study are as follows:
Steel Grade
GMW2M-ST-S Grade 4
GMW3032M-ST-S CR340
GMW3032M-ST-S CR 340 DP
GMW3399M-ST-S 1300T/1030Y M
Description
Interstitial-Free HDGI-coated mild steel
340 MPa yield strength HDGI-coated HSLA steel
340 MPa YS, 600 MPa TS HDGI-coated DP steel
1300 MPa TS cold rolled (bare) martensitic steel
The interstitial-free (IF) mild and high strength, low alloy (HSLA) steels are conventional
products that have been used in body structures for many years and are well known to be
repairable without substantial structural performance degradation by arc welding and flame
straightening. The mechanical property degradation exhibited by these materials defined a
baseline against which the degradation of AHSS properties will be judged. The dual phase (DP)
and martensitic (Mart) steels are AHSS grades used in structural applications on General Motors
vehicles scheduled to launch in the near future, components for which valid repair techniques
must be available. All steels were of approximately 1.5 mm gauge and were tested in the
longitudinal direction.
Vehicle Manufacturing Simulation
The sheet steel comprising vehicle body structure components does not exist in its as-produced
state at the time of repair, but rather has been subjected to several mechanical deformations and
thermal treatments during stamping, assembly and painting, and subsequent damage. These
treatments could alter the response of a component to subsequent repair processes. To simulate
the actual state of material at the time of repair, the mild steel, HSLA, and DP steels were first
subjected to 8% strain in uniaxial tension (to simulate part forming) and heated to 170 °C for 20
minutes to simulate paint baking. Samples of DP600 steel were also subjected to the repair
procedure thermal cycle in the as-received condition and with several combinations of low strain
and paint bake. The martensitic steel was subjected only to the paint bake treatment, as
martensitic steels rarely undergo substantial deformation during fabrication. All coated samples
were stripped of zinc to prevent environmental problems during thermal treatment and testing.
Simulated Repair Procedure Thermal Cycle
Previous General Motors studies [1, 2] measured temperature histories at various distances from
arc welds and flame straightening treatments on typical body structure components subjected to
repair. A time-temperature test matrix was developed to represent the various thermal conditions
encountered during repair welding and flame straightening, Table 1.

Table 1. Time-Temperature Test Matrix.
Hold
Hold Temperature (°C)
Time (s) 650 750 850 1000
5 X
10 X X
30 X X X X
60 X X X
90 X X
Two additional cycles were simulated. Samples were heated to 750 °C for 90 seconds, with
intermediate cooling briefly to below 538 °C after 30 and 60 seconds (to simulate multiple flame
heating cycles). Samples of IF, HSLA, and DP without prestrain and paint baking (as-received
condition) were subjected to the 650 °C, 90 second thermal treatment.
Mechanical Testing
Standard ASTM tensile tests were conducted of samples in the as-received condition, after
straining and paint baking (baking only for the martensitic steel), and after the indicated thermal
treatments. Results for two samples were averaged and yield strength (YS), ultimate tensile
strength (UTS), uniform elongation (UEL), total elongation (TEL), and n-value were reported
and plotted against temperature for each hold time. n-values were calculated in the 10% to end
of uniform elongation strain range (YS to end of uniform elongation (~3% strain) for martensitic
steel) The AC1 and AC3 temperatures (phase boundaries) for each steel grade were indicated on
the property-temperature plots.
Results and Discussion
Interstitial-Free Grade 4 Mild Steel
Tensile test results for the Grade 4 IF mild steel are shown in Figures 1 through 5. Yield strength
and TEL decreased slightly and USA and UEL increased slightly upon exposure to the lowest
temperature thermal treatment, 650 °C. Yield strength and UTS monotonically decreased and
UEL, TEL, and n-value monotonically increased upon exposure to all higher temperature
thermal treatments. Strength remained above the as-received level at temperatures up to 850 °C,
despite partial austenitization at 750 °C and 850 °C temperatures. There was no indication of
embrittlement or significant loss in ductility at the 650 °C temperature. Only temperature
affected mechanical properties; time at temperature had no significant effect.
HSLA 340 Steel
Tensile test results for the HSLA 340 steel are shown in Figures 6 through 10. Yield and
ultimate tensile strength decreased from initial strain plus age levels up to a thermal treatment of
850 °C and then increased slightly at the highest thermal treatment, 1000 °C. UEL, TEL, and n-
value all increased with increasing thermal treatment temperature up to 850 °C, and generally
decreased as temperature increased thereafter to 1000 °C. Strength remained at or above as-

IF Grade 4 Mild Steel
400
300
200
100
0
8% Strain + Bake
As Received
As Received + 650C/90s
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 1. Yield strength of IF Grade 4 mild steel after exposure to several simulated repair
thermal cycles.
IF Grade 4 Mild Steel
400
300
200
100
8% Strain + Bake
As Received
As Received +650C/90s
5sec
10sec
30sec
60sec
90sec
30x3sec
0
AC1
AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
Figure 2. Ultimate tensile strength of IF Grade 4 mild steel after exposure to several simulated
repair thermal cycles.

IF Grade 4 Mild Steel
30
25
20
15
10
5
As Received
As Received + 650C/90s
8% Strain + Bake
5sec
10sec
30sec
60sec
90sec
30x3sec
0
AC1
AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
Figure 3. Uniform elongation of IF Grade 4 mild steel after exposure to several simulated repair
thermal cycles.
IF Grade 4 Mild Steel
50
40
30
20
10
As Received
As Received + 650C/90s
8% Strain + Bake
5sec
10sec
30sec
60sec
90sec
30x3sec
0
AC1
AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
Figure 4. Total elongation of IF Grade 4 mild steel after exposure to several simulated repair
thermal cycles.

IF Grade 4 Mild Steel
0.25
0.2
0.15
0.1
0.05
0
As Received + 650C/90s
As Received
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 5. n-value of IF Grade 4 mild steel after exposure to several simulated repair thermal
cycles.
HSLA 340 Steel
600
500
400
300
200
100
0
8% Strain + Bake
As Received + 650C/90s
As Received
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 6. Yield strength of HSLA 340 steel after exposure to several simulated repair thermal
cycles.

HSLA 340 Steel
600
500
400
300
200
100
0
8% Strain + Bake
As Received
As Received + 650C/90s
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 7. Ultimate tensile strength of HSLA 340 steel after exposure to several simulated repair
thermal cycles.
HSLA 340 Steel
40
35
30
25
20
15
10
5
0
As Received + 650C/90s
As Received
8% Strain + Bake AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 8. Uniform elongation of HSLA 340 steel after exposure to several simulated repair
thermal cycles.

HSLA 340 Steel
40
35
30
25
20
15
10
5
0
As Received
As Received + 650C/90s
8% Strain + Bake
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 9. Total elongation of HSLA 340 steel after exposure to several simulated repair thermal
cycles.
HSLA 340 Steel
0.2
0.15
0.1
0.05
0
As Received + 650C/90s
As Received
8% Strain + Bake
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 10. n-value of HSLA 340 steel after exposure to several simulated repair thermal cycles.

eceived levels at temperatures up to 750 °C. There was no indication of embrittlement or
significant loss of ductility at any test temperature. As with the mild steel, only temperature had
a significant effect on mechanical properties; time at temperature had little effect.
DP 600 Steel
Tensile test results for the DP 600 steel are shown in Figures 11 through 15. Yield and ultimate
tensile strength both decreased after exposure to the lowest thermal treatment temperature, 650
°C. Yield strength continued to decrease with exposure to higher temperatures, but the UTS
increased substantially at the next higher temperature, 750 °C, then fell to an intermediate level
at the highest thermal treatment temperatures. Ultimate tensile strength fell slightly below the
as-received level when subjected to the 650 °C treatment. Yield strength remained above asreceived
levels at the 650 °C and 750 °C thermal treatment temperatures. Elongation and n-
value generally increased with increasing thermal treatment temperature. There was no
indication of embrittlement or substantial loss of ductility at any thermal treatment temperature.
As with the two previous steels, the DP 600 steel was sensitive only to temperature; time at
temperature had little effect on mechanical properties.
When the DP 600 steel in the As Received condition (without the 8% prestrain plus paint bake
thermal treatment) was exposed to the 650 °C/90 second simulated repair thermal cycle,
substantial degradation in ultimate tensile strength was noted. UTS dropped from 642 to 515
MPa, a decline of 20% and well below the 600 MPa minimum UTS specified for this grade. YS
and n-value were unchanged, and elongation improved slightly from the as-received condition.
The drop in UTS is of substantial concern in crash performance. Dual phase steels are used
increasingly in crashworthiness-critical auto body structural components because its high UTS
for a given YS provides much greater energy absorption than in conventional HSS. Many
structural components contain large areas with little or no strain imparted by the forming
process, and these areas do not benefit from strain hardening and age hardening. The
degradation in UTS from exposure to the repair thermal cycle could substantially reduce the
ability of such components to absorb crash energy. Additional tests were performed on As
Received DP 600 samples that were subjected to just the paint bake thermal cycle, and to a 1/2%
uniaxial tensile prestrain plus the paint bake cycle to determine if low levels of strain and age
hardening are sufficient to offset the strength loss from exposure to the repair thermal cycle in
DP 600 components with little or no forming strain. Results of these supplemental tests and
those of pertinent initial tests are listed in Table 2.
Table 2. Supplemental tensile tests of DP 600 steel.
Test Condition
YS
(MPa)
UTS
(MPa)
TEL
(%)
UEL
(%)
n-value
(4-6%)
n-value
(10-UEL) YPE
As Received (AR) 404 642 26 16 0.18 0.14 0.0
AR+650 °C/90sec 412 515 30 18 0.21 0.16 2.8
AR+Bake+650 °C/90sec 419 526 28 16 0.20 0.16 2.6
AR+1/2%ε + Bake 471 658 25 16 0.16 0.15 0.0
AR+1/2%ε + Bake + 650 °C/90sec 427 527 27 16 0.20 0.16 2.8
AR+8%ε + Bake 658 683 15 4 n/a n/a 0.0
AR+8%ε + Bake + 650 °C/90sec 550 610 22 11 0.13 0.11 3.5

Clearly, bake hardening and low levels of strain plus bake hardening are insufficient to offset the
UTS loss due to exposure to the lowest temperature repair thermal cycle in DP 600 steel.
Structural components of this grade with low levels of forming strain will likely exhibit UTS
well below the specified 600 MPa minimum UTS for DP 600.
Structure evolution in the DP 600 steel was studied by metallography. Figures 16 through 18
show the microstructure of the DP 600 steel in the As Received, As Received + 8% Strain + 650
°C/60s, and As Received + 8% Strain + 750 °C/90s conditions, respectively, after etching in
LePera' etchant. LePera's etchant stains the ferrite phase a light gray color and leaves
untempered martensite white. The 650 °C/60s thermal treatment substantially tempered the
martensite phase, as indicated by darkening of the martensite phase in Figure 17. The 750
°C/90s thermal treatment heated the steel back in to the ferrite plus austenite phase field and reformed
fresh untempered martensite, as indicated by the return of the light colored phase in
Figure 18.
1300 MPa UTS Martensitic Steel
Tensile test results for the 1300 MPa UTS martensite steel are shown in Figures 19 through 22.
Martensitic steels generally exhibit very little uniform elongation; only total elongation is shown
here. Yield and ultimate tensile strength were substantially reduced by all thermal treatments,
falling to about half their as-received level at the lowest thermal treatment temperature, 650 °C.
The loss in strength was accompanied by a rapid increase in elongation and n-value, particularly
as temperature entered the two-phase region (above AC1 temperature). There was, however, no
substantial embrittlement noted at any thermal treatment temperature.
General Observations
The IF mild and HSLA 340 materials are conventional steels, both relying on various
combinations of grain size, solute, and precipitation strengthening to attain their requisite
mechanical properties. These strengthening mechanisms are generally insensitive to exposure to
temperatures below the two-phase region for short times. In this study, these conventional steels
exhibited only slight decrease in strength upon exposure to the subcritical thermal treatment
temperature, 650 °C. Strength decrease in these cases is attributed to recovery of the cold work
imparted by the simulated forming step, slight grain coarsening, and in the case of the HSLA
steel, coarsening of the strengthening precipitates. Higher thermal treatment temperatures
caused these steels to enter or exceed the two-phase region, resulting in greater strength
reductions due to recrystallization and grain growth. It was encouraging to find that these steels
were not embrittled by exposure to temperatures just below the two-phase region (the so-called
blue brittleness effect). This result is consistent with previous GM work [1, 2].
The DP 600 steel is strengthened largely by the presence of islands of hard martensite in a soft
ferrite matrix. Strengthening is a function of both the volume fraction of martensite and the
strength of the martensite phase. Prior to this study, concerns had been raised that exposure to

temperatures high in the subcritical region would over-temper the martensite phase and
substantially reduce the strength of the DP 600 steel. This study confirmed that strength loss
upon exposure of the DP 600 steel to the 650 °C thermal treatment temperature was more
significant than that of the conventional steel.
A 20% reduction in ultimate tensile strength was observed DP 600 samples subjected to the 650
°C thermal treatment. Strength decrease in this material is attributed to two major factors -
tempering of the martensite and recovery of cold work from the simulated forming operation.
The ferrite in the DP 600 steel contained a much greater amount of cold work before the thermal
treatment, (strain is concentrated in the softer ferrite phase) and driving force for recovery was
much greater. Strength loss due to recovery is expected to be greater than that exhibited by the
HSLA 340 steel. Furthermore, in this study, effects of strength loss due to recovery cannot be
separated from those due to martensite tempering. For the DP 600 steel, the combined effects can
be sufficient to reduce UTS to substantially below the 600 MPa UTS minimum for the grade,
particularly in part regions that are not strained significantly by the press forming operation, and
degrade structural performance. These data indicate that the DP600 should not be repaired by
flame straightening or be exposed to heat during repair.
The 1300 MPa UTS martensitic steel is strengthened by transformation to almost 100% volume
fraction martensite. This steel was expected to exhibit significant strength loss due to tempering
of the martensite by the repair thermal treatments. Indeed, this study confirmed that exposure to
even the lowest welding and thermal straightening thermal treatments substantially degraded
mechanical properties. These results indicate that 1300 MPa UTS martensitic steel components
should not be welded or flame straightened in any manner, but be replaced in entirety if
necessitated by repair requirements. This precaution can be extended to cover all martensitic
grade steel components.
The Mild and HSLA 340 steels also exhibited slight changes in mechanical properties upon
exposure to the 650 °C thermal cycle. These changes could affect fatigue endurance, particularly
in the case of UTS reduction. However, other durability degradations introduced by the damage
and repair processes themselves are felt to be of a much greater magnitude, however, and
durability degradation due to a slight reduction in fatigue endurance is not felt to be a significant
consideration.
The lowest thermal treatment (650 °C) employed by this study is tolerated by the mild and
HSLA steels. This temperature is consistent with GM's currently recommended repair
temperature range, up to 1200 °F (650 °C - note that GM's current repair policy references
temperature in °F). This temperature is also practical to sense with the unaided eye - repair
technicians need only keep the heat below a dull cherry red color to stay within the
recommended temperature range.
GM Position on Repairability of the Subject Steel Grades
Per GM service policies, mild steel (including interstitial-free steel), high-strength-low-alloy
steel (HSLA), high tensile strength steel (HSS) and bake hardenable steel are considered
repairable after a collision. The policy supplies recommendations for the use of heat in collision

epair. The recommendation limits the maximum temperature to 1200 °F (650 °C), and the
heating time to 90 seconds. The heating can be performed two times if needed. The purpose of
this study was to re-validate these recommendations for today’s steels, and to begin evaluating
repairability of advanced high strength steels (AHSS)
The results shown here indicate that heat can be used in repair procedures for IF steel
(GM6409M, GMW2M) and 340 MPa minimum yield strength HSLA (GM6208M, GM6218M,
GMW3032M). The limitation that temperature must not exceed 1200 °F (650 °C) for a
maximum of two cycles of 90 seconds each should remain in force. The maximum temperature
is the most important factor affecting mechanical property degradation; time at temperature did
not have a significant effect on properties at the times and temperatures studied. The data also
shows that following the recommended repair thermal treatment limitations should not cause
embrittlement or significant loss of ductility in the tested materials.
These results also show that heat is not recommended for repair of dual phase materials
(GMW3032M, GMW3399M). These results show that as-received DP steels with minimum
tensile strength values of 600MPa experience a 20% drop in TS after the application of 650 °C.
Strain and bake hardening response does not counteract this behavior.
Results for the 190 KSI (1300 MPa) minimum tensile strength product (GM6123M) reinforce
the previously drawn conclusion that martensitic products of this strength level must not be
repaired. The only available repair procedure is removal and replacement.
Future Work
The industry is moving rapidly towards implementing AHSS materials with even higher strength
levels than the 600 MPa minimum ultimate tensile strength dual phase steel studied in these
experiments. Therefore, there is an urgent need to expand the range of materials tested here to
include DP 780, DP 980 and TRIP steel in the range of 600 to 800 MPa TS.
There is also a need to evaluate the combination of heat application and welding during collision
repair on the properties of high strength steels and advanced high strength steels. This is critical
because if heat cannot be used for straightening operations, there will be an additional reliance
on sectioning and welding techniques for repair of components made from AHSS materials.
Further collaborative work should be focused on these two objectives. Phase II of this study will
address both.

DP 600 Steel
800
600
400
200
0
8% Strain + Bake
As Received + 650C/90s
As Received
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 11. Yield strength of DP 600 steel after exposure to several simulated repair thermal
cycles.
DP 600 Steel
800
600
400
200
0
8% Strain + Bake
As Received
As Received + 650C/90s
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 12. Ultimate tensile strength of DP 600 steel after exposure to several simulated repair
thermal cycles.

DP 600 Steel
30
25
20
15
10
5
0
As Received + 650C/90s
As Received
8% Strain + Bake AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 13. Uniform elongation of DP 600 steel after exposure to several simulated repair thermal
cycles.
DP 600 Steel
30
25
20
15
10
5
0
As Received + 650C/90s
As Received
8% Strain + Bake
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 14. Total elongation of DP 600 steel after exposure to several simulated repair thermal
cycles.

DP 600 Steel
0.2
0.15
0.1
0.05
As Received + 650C/90s
As Received
5sec
10sec
30sec
60sec
90sec
30x3sec
0
AC1
AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
Figure 15. n-value of DP 600 steel after exposure to several simulated repair thermal cycles.

Figure 16. Microstructure of DP 600 steel in the As Received condition. LePera's etch, 1000X.
Figure 17. Microstructure of DP 600 steel after 8% strain in uniaxial tension, paint baking, and
exposure to 650 C for 60 seconds. LePera's etch, 1000X.
Figure 18. Microstructure of DP 600 steel after 8% strain in uniaxial tension, paint baking, and
exposure to 650 C for 90 seconds. LePera's etch, 1000X.

1300 MPa UTS Martensitic Steel
1600
1400
1200
1000
800
600
400
200
0
Bake
As Received
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 19. Yield strength of 1300 MPa martensitic steel after exposure to several simulated
repair thermal cycles.
1300 MPa UTS Martensitic Steel
1600
1400
1200
1000
800
600
400
200
0
Bake
As Received
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 20. Ultimate tensile strength of 1300 MPa martensitic steel after exposure to several
simulated repair thermal cycles.

1300 MPA UTS Martensitic Steel
40
35
30
25
20
15
10
5
0
As Received
Bake AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 21. Total elongation of 1300 MPa martensitic steel after exposure to several simulated
repair thermal cycles.
1300 MPa UTS Martensitic Steel
0.25
0.2
0.15
0.1
0.05
0
As Received
Bake
AC1 AC3
0 200 400 600 800 1000 1200
Hold Temperature, C
5sec
10sec
30sec
60sec
90sec
30x3sec
Figure 22. n-value of 1300 MPa martensitic steel after exposure to several simulated repair
thermal cycles.

Conclusions
1. The mechanical properties of IF grade 4 mild and HSLA 340 do not degrade substantially
upon exposure to temperatures of up to 650 °C (1200°F), for short duration.
2. The mechanical properties of the DP 600 and 1300 MPa UTS martensitic steels were
substantially degraded by exposure to 650 °C (1200°F) for any duration.
3. Exposure to 650 °C (1200°F) did not significantly embrittle any steel tested in this study.
4. Degradations of fatigue performance introduced by exposure of the steels tested to 650 °C
(1200 °F) for short duration should not be significant compared to degradations introduced
by the damage and subsequent repair processes themselves.
5. Temperature was the most significant factor affecting mechanical properties of the mild,
HSLA, and DP; properties were not significantly influenced by time at temperature.
6. Structural components made of mild, bake hardenable, solid solution-strengthened, and high
strength low alloy, can be repaired by arc welding or flame straightening processes provided
that the maximum temperature does not exceed 650 °C (1200 °F) for more than two cycles of
90 seconds each.
7. Structural components made of DP 600 and of martensitic steel of any strength should not be
repaired by application of heat (flame straightening), but replaced in entirety.
Recommendation
Following the results of Phase I of this study, the GM repair matrix is updated according to the
recommendation below. In this update, consideration is given to higher strength grades of dual
phase steel and other AHSS types referenced by GMW3399M-ST-S, grades that may be of
interest in future studies.
Recommended GM Steel Repairability Matrix
Grade Repairable Use of Heat Temp. Range Maximum Heat
Mild Steel Yes Yes Up to 1200 °F 90 sec. x 2
(650 °C)
Bake Hardenable Yes Yes Up to 1200 °F 90 sec. x 2
(650 °C)
Solid Solution- Yes Yes Up to 1200 °F 90 sec. x 2
Strengthened
(650 °C)
High Strength, Yes Yes Up to 1200F 90 sec. x 2
Low Alloy
(650 °C)
Dual Phase
TBD No N/A N/A
600 MPa UTS
TRIP TBD TBD TBD TBD
Martensitic No No N/A N/A

References
1. E. G. Brewer, K. Malstrom, R. Stevenson, and H. D. Pursel, "Effect of Simulated Repair
Heat Treatments on the Physical Properties of High Strength Steels," General Motors
Corporation Research Report No. PH-1251, August 9, 1985.
2. R. Stevenson, E. G. Brewer, K. Malstrom, and H. D. Pursel, "Effect of Simulated Repair
Heat Treatments on the Physical Properties of High Strength Steels," SAE Technnical Paper
No. 910292, SAE, Warrendale, PA, 1991.