September 11 – 12, 2018

Forging Industry Technical Conference

Long Beach, CA

Presenters

company logo

should fit within

this space

High Strength, High Toughness Microalloyed Steel Forgings

Produced with Relaxed Forging Conditions and No Heat Treatment

Aaron E Stein (aes151@pitt.edu) and Dr. Anthony J. DeArdo (deardo@pitt.edu)

Basic Metals Processing Research Institute, Department of Mechanical

Engineering and Materials Science, University of Pittsburgh

Presenters

company logo

should fit within

this space

Introduction

Purpose and goals of project

Background

Overview of RCF (Recrystallization Controlled Forging)

Derivation from RCR (Recrystallization Controlled Rolling)

Composition

Experiments

Procedures

Purposes and Reasons

Results

Laboratory and full scale results

Discussion

Conclusion

Agenda

Presenters

company logo

should fit within

this space

Project Purpose

Develop strong, tough and economical microalloyed steel forgings

Submit V-Ti-N steel system to RCF in forging applications

Project Goals

One steel three strength and toughness combinations

– Thermomechanical processing alterations

– Cooling path changes

Investigate V-Ti-N compatibility with conventional forging practice

Introduction

Presenters

company logo

should fit within

this space

Microalloyed, high strength forging steels

QT steels = Expensive

RCF steels = Economical

Prior austenite grain refinement

TiN anchors PAGB (Prior Austenite Grain Boundary)

Repetitive recrystallization decreases PAGS (Prior Austenite Grain Size)

– Operating Window: TGC (Grain Coarsening Temperature) to T95 (Recrystallization Temperature)

– Recrystallization without subsequent coarsening

PAGS Final Transformed Microstructure

Cooling Microstructures

– IAC and IDQ

– WET (Water End Temperature/Quench End Temperature)

Titanium Grain size control (High temperature TiN)

Vanadium Precipitation strengthening (Polygonal ferrite only)

Background

Presenters

company logo

should fit within

this space

Controlled cooling produces 3 strength levels

Yield strength 60 Ksi (Slow cooling)

– Ferrite pearlite microstructure

Intermediate strength 90 Ksi (Fast cooling to below BS with short hold)

– Bainite microstructure

– ~30°C/s to 450-550°C and hold

– ~54 °F/s to 842°F-1022°F

High strength 100-120 Ksi (Fast cooling)

– Martensite/Auto-tempered martensite microstructure

– Water quenched to room temperature (WQRT)

Background

Presenters

company logo

should fit within

this space

Figure 1: Pinning of Austenite Grain Boundaries by TiN Particles (Arrowed) and

Ti Peak in the EDS Spectrum Taken From a Particle (SEM Micrograph)1

TiN Particles Anchoring Prior Austenite Grain Boundaries

Background

1Roy et al

Presenters

company logo

should fit within

this space

Figure 2: Compositions for V-Ti-N Systems2

Background

Figure 3: Reheated Grain Size

Coarsening2

2Zheng et al

Presenters

company logo

should fit within

this space

Background

Figure 4: Grain Coarsening Rates for V-Ti-N

systems2

Figure 5: Austenite Grain

Size vs Time for V-Ti-N

Systems2

Grain Coarsening for 4 Vanadium Steel

Systems

Very slow coarsening of AGS at 1050°C (1922°F)

2Zheng et al

ASTM 4

ASTM 4.4

ASTM 7.3

Presenters

company logo

should fit within

this space

In Figure 6, the red line denotes the

separation between recrystallization and

non-recrystallization after deformation

V-Ti-N exhibits complete recrystallization down to

approximately 860°C (1580°F)

Also exhibiting lower die wear

Background

Figure 6: Grain Recrystallization for V-Ti-N systems2Figure 7: Effect of N on TGC, TRX, and

the RCF Operating Window2

2Zheng et al

RCF

Operating

Window

TRH

TF

Presenters

company logo

should fit within

this space

RCR Forging Practice

The 10V40 passes occur between 1075°C (1967°F) and 1000°C (1832°F)

– Directly in the theorized region of RCF processing for the V-Ti-N system,

approximately centered between TGC and T95

Background

Figure 8: Compositions for V-Ti-N

Systems3

3DeArdo & Hua

Low Die

Wear

High Die

Wear

Presenters

company logo

should fit within

this space

Table 1: Composition and Ideal Diameter of RCF Project Steels

Steel Compositions

Element/St

eel

M1 M2 M3 10V40 T1 T2

C (wt%) 0.10 0.10 0.10 0.37 0.15 0.20

V (wt%) 0.06 0.06 0.12 0.060 0.080 0.11

Ti (wt%) 0.015 0.015 0.015 - 0.003 0.003

N (wt%) 0.012 0.012 0.012 0.0094 0.009 0.009

Cr (wt%) 0.50 0.25 0.50 0.10 0.10 0.10

Mo (wt%) 0.30 0.15 0.30 0.02 0.030 0.030

Mn (wt%) 1.20 1.20 1.20 1.14 1.35 1.50

Si (wt%) 0.40 0.40 0.40 0.22 0.20 0.30

P (wt%) 0.010 0.010 0.010 0.010 0.010 0.010

Al (wt%) 0.030 0.030 0.030 0.028 0.030 0.030

Di (in) 1.5161 0.8566 1.6526 1.571 0.804 1.346

Presenters

company logo

should fit within

this space

Figure 9: M1 CCT Diagram with Interrupted Cooling

Temperatures WET1 (α-P), WET2 (αB) and WET3 (α’)

JMATPro Analysis

WET1

WET3

WET2

M50

Ms

WET2

Presenters

company logo

should fit within

this space

JMATPro Analysis

Figure 10: (Left) M1 Phase Diagram Exhibiting Bainite (Blue) and Ferrite (Orange), and (Right)

M1 Phase Diagram Exhibiting Martensite (Black) and Bainite (Blue)

Phase-Temperature Simulation Curves

Used to determine ideal cooling paths for cooling experiments

1°C/s (1.8°F/s) 30°C/s (54°F/s)

Presenters

company logo

should fit within

this space

Figure 11: Laboratory Experiments Displayed

on Thermomechanical Processing Route

Experiment

Groupings

Group 1

– Grain coarsening reheat

studies in furnace

Group 2

– Recrystallization

deformation studies in

MTS

Group 3

– Phase transformation

cooling studies and

quench tank

Results - Laboratory

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300 350 400

Tem

per

atu

re (

°C)

Time (s)

Forging Process: MTS Simulation

1 2

3

Presenters

company logo

should fit within

this space

Figure 12: Steel M1 Grain Coarsening Curve, Showing TGC at 1150°C (2102°F)

Grain Coarsening Reheat Experiments

Determine TGC for each steel

In this study TGC is defined as the temperature where largest grain averages

deviate from the average grain averages by a significant amount

– The curve below shows this demarcation at the 1150°C (2102°F) temperature for steel M1

– UA Diam – Average equivalent diameter of grains larger than two standard deviations above

to grain average ; AVG Diam – Average equivalent grain diameter

Results - Laboratory

Presenters

company logo

should fit within

this space

Table 2: TGC and TRH for Control Steel 10V40 and BAMPRI Steels M1 - M3

Grain Coarsening Reheat Experiments

TGC determined for each steel and displayed in table 2 below

– TRH (Reheat Temperature) for further experiments set at 25°C (45°F) below TGC

Results - Laboratory

Steel TGC (Grain Coarsening

Temperature)

TRH (Suggested Reheat

Temperature)

M1 1150°C (2102°F) 1125°C (2057°F)

M2 1150°C (2102°F) 1125°C (2057°F)

M3 1200°C (2192°F) 1175°C (2147°F)

10V40 1050°C (1922°F) 1025°C (1877°F)

Presenters

company logo

should fit within

this space

Figure 13: Micrograph of Steel M3 After Deformation at 775°C (1427°F),

1000X Magnification, Showing Majority Non-Recrystallized Microstructure

Recrystallization Deformation Studies

Determine T95 for each steel

– Temperature at which 95% recrystallization occurs, defines the lower limit for

deformations in the RCF process

– Determined from ImageJ analysis of quenched and picric etched micrographs such as

figure 13, which is well below T95, causing pancaked structure and high forging loads

Results - Laboratory

Deformation

Axis

100µm

Presenters

company logo

should fit within

this space

Table 3: T95 Temperature For Each BAMPRI Steel accompanied by the

Equivalent Diameter Grain Size For the T95 Deformation Temperature

Recrystallization Deformation Studies

T95 temperature determined for each steel

– 900°C (1652°F)

– Guaranteed complete recrystallization with largest grain refinement

– Steels for further experiments use suggested reheat temperature, with

deformation at 900°C (1652°F) followed by water quench to room temperature

Results - Laboratory

Steel T95 T95 Eq Diam. (µm)

M1 850°C (1562°F) 12.05 (9.5 ASTM)

M2 800°C (1472°F) 10.73 (9.8 ASTM)

M3 850°C (1562°F) 8.24 (10.6 ASTM)

Presenters

company logo

should fit within

this space

Phase Transformation Cooling Studies

Investigated cooling paths to derive specified microstructures

– 3 cooling paths investigated for each of the 3 steels

– ACRT

– WQRT

– Cool 30°C/s (54°F/s) to 500°C (932°F) and hold for 110s

– microstructures intended to be produced, respectively

– α-P microstructure

– α' microstructure

– αB microstructure

Hardness values calculated using 300gf Vickers Hardness indentations

– Ultimate tensile strengths estimated via 3.2 x VHN value

– Proposed in previous BAMPRI unpublished research

Results - Laboratory

Presenters

company logo

should fit within

this space

Phase Transformation Cooling Studies

Ferrite samples underwent cooling using still air cool to room temperature

Bainite samples underwent cooling using forced Helium convection to 500°C (932°F)

– Followed by 110s hold, and water quench to room temperature

Martensite samples underwent cooling using water quench to room temperature

Results - Laboratory

Table 4: Tested Cooling Paths and Results for Various Conducted Cooling Experiments

M1 – 0.06 V M3 – 0.12 V

Phase α αB α' α αB α’

Cool Rate ACRT 30°C/s

(54°F/s)

WQRT ACRT 30°C/s

(54°F/s)

WQRT

WET RT 500°C

(932°F)

RT RT 500°C

(932°F)

RT

VHN 216.1 243.1 444.0 234.0 269.07 437.8

UTS (Converted) 692MPa

(100Ksi)

778MPa

(113Ksi)

1421MPa

(206Ksi)

749MPa

(109Ksi)

861MPa

(125Ksi)

1401MPa

(203Ksi)

Phase % 57.98 100 100 64.19 100 100

Presenters

company logo

should fit within

this space

Boiling H2O Quench Tank

Unit designed to cool forging at accelerated rate

– Faster than forced convection, slower than traditional

quenches

– Submersed in boiling water to controllably cool

samples to 550°C (1022°F)

Deemed SIQU

– Submersion Interruptive Quench Unit

– Soon to be implemented at MFC

Results - Laboratory

Table 5: Cooling Rates Through temperature

Ranges in the Quench Tests

Figure 14: SIQU (Submersion

Interruptive Quench Unit)

Temperature

Range

950-500 (°C) 800-500 (°C) 600-450 (°C)

Center

Cooling Rate

7.65°C/s

(13.77°F/s)

8.80°C/s

(15.84°F/s)

14.18°C/s

(25.52°F/s)

Quarter

Cooling Rate

7.56°C/s

(13.61°F/s)

8.84°C/s

(15.91°F/s)

14.16°C/s

(25.49°F/s)

Presenters

company logo

should fit within

this space

SIQU Timed Submersion Quench Tests

Results - Laboratory

Figure 15: SIQU Trials for Acquisition of Ideal Bainite

Formation Path

Note: The times in the

boxes refer to the

submersion time of the

piece in the boiling water

quench

Presenters

company logo

should fit within

this space

Initial Forging Trials on Steels M1, M2 and M3

Standard 10V40 two pass deformation schedule

Three cooling paths

– ACRT

– Fast ACRT

– WQRT

Each sample analyzed with various standard metallurgical procedures

Charpy and tensile samples currently under machining

Results - Initial Forging Trials

Presenters

company logo

should fit within

this space

Initial Full Forging Specimen Sectioning

Section 1 – Mid-Flange ; Section 2 – Fillet ; Section 3 – Center

Column ; Section 4 – Quarter Column ; Section 5 - Lip

Results - Initial Forging Trials

Figure 16: Sample Diagram for Initial Tested Forging Pieces

Presenters

company logo

should fit within

this space

UTS Values Calculated From Measured VHN (300gf) Data for

Five Locations

A – ACRT

B – Fast ACRT

C – WQRT

Each location = Average of three measurements

Results - Initial Forging Trials

Table 6: Calculated UTS Values (Mpa) for Initial Full Forgings

M1A M1B M1C M2A M2B M2C M3A M3B M3C

1 751 751 1298 663 680 1149 786 912 1237

2 683 922 1069 660 618 1090 814 854 1145

3 632 702 1014 602 617 1047 728 873 967

4 731 793 1007 650 666 963 748 798 993

5 652 890 1420 547 644 1280 698 867 1458

Average 690

(100 Ksi)

812

(118 Ksi)

1162

(169 Ksi)

624

(91 Ksi)

645

(94 Ksi)

1106

(160 Ksi)

755

(110 Ksi)

861

(125 Ksi)

1160

(168 Ksi)

Presenters

company logo

should fit within

this space

Ferritic Grain Sizes for Initial Full Forgings

Results - Initial Forging Trials

M1A M1B M2A M2B M3A M3B

1 10.12 8.06 8.42 8.36 9.84 8.05

2 8.42 8.39 12.03 7.35 8.25 7.68

3 14.31 12.55 11.96 11.14 11.11 11.10

4 11.50 10.13 11.38 12.66 11.89 10.79

5 11.35 8.15 9.30 8.56 8.85 8.01

Average 11.14

(9.7 ASTM)

9.46

(10.2 ASTM)

10.62

(9.8 ASTM)

9.61

(10.1 ASTM)

9.99

(10 ASTM)

9.13

(10.3 ASTM)

Table 7: Equivalent Ferritic Grain Diameter (µm) for Initial Full Forgings

For

Reference:Grain

Diameter

(µm)

ASTM

Grain

Size

Number

16.8 8.5

14.1 9

11.9 9.5

10.0 10

8.41 10.5

7.07 11

Table 8: Metric to

Imperial Conversions

for Grain Size

Measurements

Presenters

company logo

should fit within

this space

Discussions

Elevated grain coarsening temperatures

– Evidence of TiN particles

– TEM results will prove this

Grain refinement shown through deformations

– Recrystallization effective in grain size reduction

Cooling experiments provide information

– Possible to obtain predominantly ferrite, bainite, and martensite microstructures

Quench tank

– Experiments provide proof of concept for intermediate strength level production

Initial Full Forgings

– Hardness values and calculated strengths higher than 10V40

– Strength increase with 0.3% reduction in Carbon

» Expected increase of toughness values

Discussions

Presenters

company logo

should fit within

this space

The RCF steels exhibit grain coarsening temperatures of 1150°C

(2102°F) and 1200°C (2192°F), compared to the 1050°C

(1890°F) grain coarsening temperature for the 10V40 steel.

Deformation experiments demonstrate that the RCF steels can

be refined to approximately 10µm equivalent diameter grain size.

The thermomechanical processing routes investigated

demonstrate the capability of obtaining multiple strength and

toughness combinations from each steel.

Initial full scale forgings of the steels show increased hardness

and calculated tensile values over those of the 10V40 steel.

These values range from about 200 to 400 VHN, and about 700

to 1400 Mpa (100 to 203 Ksi).

Conclusions

Presenters

company logo

should fit within

this space

BAMPRI would like to extend thanks and acknowledgements to

University of Pittsburgh and Mechanical Engineering and Materials Science

Department for use of facilities

Forging Industry Education and Research Foundation for sponsorship of the

project

Meadville Forging Company for cooperation and advice from industry

experts

TIMKENSTEEL Steel Company for generous supply of experimental lab

heats for the steels in the project

The author extends personal thanks to

Dr A. J. DeArdo and Dr. M. J. Hua, BAMPRI

Ms. Lewis, FIERF

Mr. McLean and Mr. Geib, MFC

Mr. Zorc, TIMKENSTEEL Steel Company

Acknowledgements

Presenters

company logo

should fit within

this space

Thank you and enjoy the rest of the conference.

Thank You

Presenters

company logo

should fit within

this space

Presenters

company logo

should fit within

this space

Table 9: Cooling Schedules to Produce Various Microstructures

Using previous charts, the following cooling schedules were

devised to produce microstructures of a predominant phase

CR – Cooling Rate ; WET – Water End Temperature ; Phase –

Percentage Present of Indicated Phase

JMATPro Analysis

M1 M2 M3 10V40 T1 T2

α-PCR (°C/s) 0.1 0.5 0.1 1 1 0.5WET (°C) 600 540 600 550 550 575

Phase (%) 77-23 68-32 74-26 24-76 50-36 45-55

αB

CR (°C/s) 10 10 10 10 30 10WET (°C) 425 485 400 350 400 460

Phase (%) 78 63 71 81 78 78

α'CR (°C/s) 30 30 30 30 - 30WET (°C) 175 225 175 150 - 280

Phase (%) 69 15 39 61 - 18

Presenters

company logo

should fit within

this space

1

Roy, S., D. Chakrabarti, and G. K. Dey, "Austenite grain structures in Ti-and

Nb-containing high-strength low-alloy steel during slab reheating,“

in Metallurgical and Materials Transactions A 44.2 (2013): 717-728.

2

Y. Z. Zheng, G. T. Tang and Z. H. Lin, "Precipitation, Recrystallization and

Transformation in V--Ti--N Microalloyed Steels," in HSLA Steels:

Processing, Properties and Applications, Warrendale, 1990.

3

A. J. DeArdo and M. Hua, "Some Comments on the Physical Metallurgy of

HSLA Steels Containing Vanadium and Nitrogen," in Materials Science and

Technology 2014, Warrendale, 2014.

References