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High Strength Forged Steel and Process Design
Piyada Suwanpinij, TGGS, [email protected]
Metallurgy for Forging Process Design and Tool Life Improvement vs. XRD Forum
Wed 29 January 2020
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Outline
State of the Art
• Products from Microalloyed Forged Bars
• Microstructure of Air-Cooled Microalloyed Forged Steels
Method
• Industry-near Forging Experiment
• Metallography: LOM for phase fraction and grain size
2
• Mechanical Properties: tensile testing, hardness, impact
• Fraction of Precipitates: Synchrotron XAS
Results and Discussion
• Cooling Curves with Recalescence
3
• Calculated Contribution from each Strengthening Mechanism
• Strength of Steel and Strengthening Mechanisms
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Microalloyed Forged Steels
Typical Applications in Automotive Industry
■Crankshaft
■Connecting rod
■Steering link
■Axle leg
■Wheel carrier
■Control arm
■Piston for diesel engines
■Common rail for diesel engines
…
3
•Source: Basic Knowledge Forgings – Significance, Design, Production, Application, Industrieverband Massivumformung
Wheel Carrier
Piston
Crankshaft
Connecting rod
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Microalloyed Forged Steels
■ Quenched and tempered steel (Q&T) needs an additional heat treatment
(left picture)
■ Microalloyed steels achieve final properties during controlled cooling
directly after forging (right picture)
■ Shorter process chain compared to Q&T steels No costs for additional
heat treatment
■ No risk for crack formation during quenching
■ Low cost alloying concept
■ High strength
■ Good machinability
4
Die forging
Controlled cooling
Time
Tem
pera
ture
Tem
pera
ture
Time
Die forging
Hardening
TemperingStress relief
Str
aig
hte
nin
g
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Microalloyed Forged Steels
38MnVS6 as an example for a typical steel and its
transformation behavior
5
C Si Mn P S Cr Al V N
0,38 0,58 1,39 0,016 0,056 0,16 0,048 0,11 0,017
Chemical
Composition in wt.-%
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Microalloyed Forged Steels - Adjusting Phase Fractions
6
18MnVS5
38MnVS4
55MnVS3
70MnVS4
38MnVS6
Source: H. Dickert et al, Steels in Cars and Trucks 2017
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Strength of Different Matrices according to Pickering
7
Martensite Bainite
Ferrite,
Pearlite
Transformation temperature in °C
Te
ns
ile
str
en
gth
in M
Pa
1200
1050
900
750
600
450
300 400 500 600 700 800
Ref.: F.B. Pickering, in Transformation and
Hardenability in Steels, Climax
Molybdenum Co., Ann Arbor, Michigan,
1967, p. 109.
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Contribution to Yield Strength in Mild and HSLA Steels
8
Ref. :M. Cohen and S.S. Hansen,
Microstructural Control in Microalloyed
Steels, in MiCon78: Optimization of
Processing, Properties, and Service
Performance Through icrostructural
Control, ASTM STP 672, H. Abrams,
G.N. Maniar, D.A. Nail and H.D.
Solomon, Ed., 1979, p 34–52
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Precipitation Hardening: Cutting vs. Looping
9
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Precipitation Hardening in Ferrite-Pearlite
10
Martensite Bainite
Ferrite,
Pearlite
Transformation temperature in °C
Te
ns
ile
str
en
gth
in M
Pa
1200
1050
900
750
600
450
300 400 500 600 700 800
Aged Hardening
Ferritic Pearlitic
Steels by V(C,N)
𝑌𝑆
= 𝑓𝛼 35 + 58 %𝑀𝑛 + 17.4 𝐷𝛼−0.5
+ 1 − 𝑓𝛼 178 + 3.8 𝑝−0.5
+ 63 %𝑆𝑖 + 425 %𝑁 −0.5 + ∆𝑌𝑆𝑝
∆𝑌𝑆𝑝,𝑂𝑟𝑜𝑤𝑎𝑛−𝐴𝑠ℎ𝑏𝑦
)
S. Zajac, T. Siwecki, W.B. Hutchinson and R.
Lagneborg: Strengthening Mechanisms in Vanadium
Microalloyed Steels Intended for Long Products, ISIJ
international 38.10 (1998), pp. 1130-1139
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Method: Industrial Cast and Forging Experiments
11
C Si Mn P S Cu Al Cr Mo Ni N V Ti
0.37 0.59 1.05 0.013 0.025 0.11 0.016 0.17 0.04 0.11 0.013 0.268 0.018
350 mm.
125 mm.
Ø 85 mm.
Ø 30 mm.
Ø 50 mm.
Ca. 1000 mm.
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12
■Final Cooling on the conveyor belt is controlled by:
■Speed of the conveyor belt
■Use of fans (to increase cooling rate)
■Covering the conveyor belt (to reduce the cooling rate).
Industrial Process
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Method: Industry-near Process
■Open die manual forging on a forging hammer
■Controlled cooling by using different pressures of compressed air
■Temperature measurement with a pyrometer
13
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Results: Cooling Curves with Recalescence
14
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Results: Cooling Curves with Recalescence
15
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Ø 30 mm.
Air Flow
F 35%, P 65%, F 25%, P 70%, AF 5% F 20%, P 72%, AF 8%
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Ø 50 mm.
F 35%, P 65%, F 30%, P 67%, AF 3% F 25%, P 70%, AF 5%
Air Flow
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100 micrometer
Ø 30 mm. 2.5 bar
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Ø 30 mm. still air
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Calculation of Strengthening Factors
𝑌𝑆
= 𝑓𝛼 35 + 58 %𝑀𝑛 + 17.4 𝐷𝛼−0.5 + 1 − 𝑓𝛼 178 + 3.8 𝑝
−0.5
+ 63 %𝑆𝑖 + 425 %𝑁 −0.5 + ∆𝑌𝑆𝑝
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=7
∆𝑇
Calculation of the Pearlite Interlamellar Spacing
Mehl and HagelBleck
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Results: Cooling Curves with Recalescence
23
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Results: Cooling Curves with Recalescence
24
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ቍ𝐿𝑉𝐶 =2
3
𝜋
𝑓− 2 ∗ 𝑟𝑉𝐶 (𝑚
Calculation of the Precipitation Hardening
𝑌𝑆
= 𝑓𝛼 35 + 58 %𝑀𝑛 + 17.4 𝐷𝛼−0.5 + 1 − 𝑓𝛼 178 + 3.8 𝑝
−0.5+ 63 %𝑆𝑖
+ 425 %𝑁 −0.5 + ∆𝑌𝑆𝑝
ቇ∆𝑌𝑆𝑝,𝑂𝑟𝑜𝑤𝑎𝑛−𝐴𝑠ℎ𝑏𝑦 =0.8𝑀𝐺𝑏
2𝜋 1 − 𝜈𝐿𝑉𝐶l n(
𝑥
2𝑏) (𝑀𝑃𝑎
)
)
M – Tailor Factor, G – Shear modulus, b – Burger vector, - Poison’s ratio
𝑥 = 22
3𝑟𝑉𝐶
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YS
UTS
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YS
UTS
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1. Free Nitrogen 2. Precipitation hardening
■Too difficult (for me ;-))
■Theoretically by measuring volume
and average radius of the
precipitates for the mean distance
between particles
The 2 Unknown Strengthening Fractions
∆𝑌𝑆𝑝,𝑂𝑟𝑜𝑤𝑎𝑛−𝐴𝑠ℎ𝑏𝑦
=0.8𝑀𝐺𝑏
2𝜋 1 − 𝜈𝐿𝑉𝐶l n(
𝑥
2𝑏) (𝑀𝑃𝑎)
ቍ𝐿𝑉𝐶 =2
3
𝜋
𝑓− 2 ∗ 𝑟𝑉𝐶 (𝑚)
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Calculation of Precipitates Fraction vs. Dissolved Fraction
Synchrotron XAS for the Precipitation Fraction
Absorption Edge by
Photoelectric
Absorption – XANES –
defines central atom
type
5465 eV for K-shell binding energy in V
𝜒 𝑘 =
𝑓
𝑆02𝑁𝑗
𝑘𝑅𝑗2 𝑓𝑗
𝑒𝑓𝑓𝑘, 𝑅𝑗 sin 2𝑘𝑅𝑗 + 𝜑𝑗 𝑘 𝑒−2𝜎𝑗
2𝑘2𝑒−
𝑅𝑗𝜆 𝑘
Interference between
Emitted Photoelectron
and Scattered X-Ray
from Surrounding
Atoms – EXAFS –
defines type of
surrounding atoms
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XAS Spectra on the Samples and Standard
30
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Calculation of Precipitates Fraction vs. Dissolved Fraction
31
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Results on XAS Fractions for All Samples
32
Ø 50 mm
Ø 30 mm
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Hooray!!! All Terms can be Solved!!!
𝑌𝑆
= 𝑓𝛼 35 + 58 %𝑀𝑛 + 17.4 𝐷𝛼−0.5 + 1 − 𝑓𝛼 178 + 3.8 𝑝
−0.5+ 63 %𝑆𝑖
+ 425 %𝑁 −0.5 + ∆𝑌𝑆𝑝
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34
YS
UTS
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YS
UTS
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Conclusions
36
Forged Components can be Forged and Air Cooled Without
Heat Treatment
Extra Ordinary Strength from Conventional Composition,
mainly from Pearlite
Process Window for Different Bar Sizes can be Determined
27-40% of Vanadium is still in the Solid Solution
At Highest Cooling Rate (Highest Air Pressure), Lowest
Fraction Contributed by Precipitation Hardening
The Methodology can be Followed in the Industry – Both
Production and R&D6
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Acknowledgement
GEORGSMARIENHÜTTE GMBH - WWW.GMH-GRUPPE.DE/DE-EN/
37