Torsional Fatigue Performance of Induction Hardened 1045 and 10V45 Steels

Lee Rothleutner, Robert Cryderman, and Chester J. Van Tyne George S. Ansell Dept. of Metallurgical and Materials Engineering,

Colorado School of Mines, Golden, CO 80401 USA lrothleu@mines.edu

Jody Burke

Gerdau Special Steel North America, Jackson, MI 49201 USA

Abstract

Microalloying of medium carbon bar steels is a common

practice for a number of traditional components; however, use

of vanadium microalloyed steels is expanding into

applications beyond their original designed use as controlled

cooled forged and hot rolled products and into heat treated

components. As a result, there is uncertainty regarding the

influence of vanadium on the properties of heat treated

components, specifically the effect of rapid heat treating such

as induction hardening. In the current study, the torsional

fatigue behavior of hot rolled and scan induction hardened

1045 and 10V45 bars are examined and evaluated at effective

case depths of 25, 32, and 44% of the radius. Torsional fatigue

tests were conducted at a stress ratio of 0.1 and shear stress

amplitudes of 550, 600, and 650 MPa. Cycles to failure are

compared to an empirical model, which accounts for case

depth as well as carbon content.

Introduction

Induction hardening of medium-carbon steels in the ferrite-

pearlite condition is common in applications such as

automotive shafts and forgings. The influence of vanadium

additions on the microstructure, mechanical properties, and

fatigue properties of wrought medium-carbon ferrite-pearlite

steels is well documented in literature [1-7]. However, only a

few studies have investigated the role of vanadium on

induction hardened component microstructures [8-11]. No

studies have investigated the direct influence of vanadium on

fatigue performance. A fundamental understanding of

vanadium’s role in microstructural evolution during induction

hardening and its effect on the fatigue performance of

medium-carbon steel will allow further optimization of

processing and alloy design, potentially improving component

performance.

Torsional Fatigue

Three torsional fatigue studies of induction hardened medium-

carbon steels were found in literature that provided insight for

the current study. Hurd [12] studied both smooth and splined

20 mm diameter induction hardened shafts at case depths of 3,

4, and 7 mm. Although two alloys were used in the study,

SAE 1050H and 38B3, only data from the 1050H was

discussed in the paper. Hurd found that fatigue failures, for all

case depths investigated, were surface initiated at high shear

stress amplitudes. In these surface initiated specimens,

initiation and early crack growth were observed in Mode III

on the order of 1 mm in length and occurred along

longitudinal shear planes which were followed by a Mode I,

normal stress, crack propagation to failure. The longitudinal

crack growth is believed to be a result of linking micro-cracks

initiated at inclusions in the crack plane [12]. At low shear

stress amplitudes, Hurd observed the lowest case depth

condition initiated failure sub-surface at the case-core

transition then propagated in a variety of complex growth

modes. The second study examined was by

Ochi et al. [13, 14]. Ochi et al. studied three plain-carbon

steels (0.35, 0.41, and 0.54 wt. % C) and of each alloy, three

case depths were examined ranging from 2.1 to 6.8 mm in a

20 mm diameter test specimen. Ochi et al. observed similar

results to those found by Hurd in that high shear stress

amplitudes resulted in surface initiated failure while low

amplitudes initiated failure sub-surface. However, Ochi et al.

observed a shift in initiation from surface to sub-surface that

was not only a function of case depth but also a function of

carbon content. As case depth and shear stress amplitude

decreased for a given carbon content, failure initiation shifted

from surface to sub-surface. This change is in agreement with

the observations made by Hurd [12]. In the third study,

Cryderman et al. [15] looked at the influence of continuous

cast section size on the torsional fatigue performance of

induction hardened SAE 1050. Four slight variations in carbon

content were tested (0.52 to 0.56 wt. % C) at a nominal case

depth of 4.4 mm in a 12.8 mm diameter test specimen.

Although negligible differences between the torsional fatigue

performance of the steels were observed, differences in

initiation mode were noted. At high shear stress amplitude

initiation occurred along longitudinal shear planes, Mode III,

while at low strains, or low shear stress amplitudes, initiation

occurred due to normal stresses, Mode I. Table 1 shows a

summary of the aforementioned torsional fatigue studies

regarding carbon contents and effective case depths examined.

One important note about the three studies shown in Table 1

regards the stress ratio. Two of the studies did not report the

stress ratio of the torsional fatigue data. Nevertheless, a study

by Hurd and Irving [16] showed that stress ratio does not play

a significant role in torsional fatigue life. Figure 1 shows all

159 observations of torsional fatigue data for the conditions

summarized in Table 1. The data span a very broad range of

Proceedings of the 28th ASM Heat Treating Society Conference October 20–22, 2015, Detroit, Michigan, USA

Copyright © 2015 ASM International® All rights reserved

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600

cycles to failure, 100 to 3 million cycles, and shear stress

amplitude, 400 to 1200 MPa.

Table 1: Induction Hardened Torsional Fatigue Data

Reference C (wt. %)

Normalized

Effective

Case Depth, t/r

Stress

Ratio

Hurd [12] 0.50 0.30 / 0.40 / 0.70 NR

Ochi

et al. [13]

0.35

0.41

0.54

0.21 / 0.34 / 0.48

0.25 / 0.42 / 0.62

0.37 / 0.52 / 0.68

NR

Cryderman

et al. [15]

0.52

0.54

0.56

0.54

0.33

0.34

0.35

0.35

-1

Figure 1: Torsional fatigue life data (finite only) of induction

hardened medium carbon steel shafts from the

literature [12, 13, 15].

Experimental Procedures

Materials

Two grades of SAE 1045 were produced for this research, one

with aluminum additions and one with vanadium additions,

designated as 1045 and 10V45 respectively. Both alloys were

continuous cast as separate industrial heats and hot rolled to a

final diameter of 39.7 mm, giving a reduction ratio of

18.8 to 1. Table 2 shows the nominal chemical composition

for both alloys. Sulfur levels are much lower than most

commercially produced SAE 1045 heats to reduce, and

possibly eliminate, fatigue crack nucleation at inclusions

during fatigue testing. Slight differences in chemistry, other

than aluminum, vanadium, and nitrogen, are observed between

the 1045 and 10V45. The 10V45 has slightly higher carbon,

manganese, and silicon levels. As a result, ideal diameter (DI)

calculations differ between the two steels, 36.0 mm for the

1045 and 45.5 mm for the 10V45 (39.9 mm without including

the vanadium), and will be considered during the analysis of

the results [17].

Table 2: Chemical Composition of Materials in wt. %

Steel C* Mn Si Ni Cr Mo

1045 0.44 0.74 0.23 0.10 0.12 0.03

10V45 0.47 0.82 0.28 0.09 0.12 0.03

Steel V Al N S P Cu

1045 0.002 0.016 0.0068 0.006 0.010 0.26

10V45 0.080 0.007 0.0100 0.009 0.007 0.22

* Standard deviation of 0.01 wt. % (n=5) for each steel.

Figure 2 shows representative secondary electron SEM images

of the two materials in the hot-rolled condition taken from the

mid-radius of the round bar and transverse to the rolling

direction. Microstructures are ferrite-pearlite with the 10V45

having markedly smaller ferrite grain size and ferrite fraction.

The 1045 has 16.9 ± 2.0 % ferrite with a ferrite grain size of

6.4 ± 3.9 µm while the 10V45 has 13.5 ± 1.5 % ferrite with a

ferrite grain size of 3.3 ± 2.8 µm. These differences in

microstructure, as well as the vanadium additions to the

10V45, result in large differences in hardness, 217 ± 5 HV1kg

for the 1045 and 281 ± 9 HV1kg for the 10V45.

(a) (b)

Figure 2: Representative secondary electron SEM images of

as-hot rolled (a) 1045 and (b) 10V45 steels.

Specimen Design

Both torsional fatigue and induction hardening requirements

were considered in designing the test specimen for the current

study. Figure 3 shows a schematic of the torsional fatigue

specimen. The shallow U-shaped groove torsional fatigue

specimen design provides minimal stress concentration (<1%)

and results in a region approximately 15 mm long in the center

of the specimen that is within 95% of maximum shear stress.

This design allows a reasonable volume of material to be

fatigue tested while facilitating induction hardening by both

scanning and single-shot methods. Specimens were machined

from the center of the as-hot rolled 1045 and 10V45.

Figure 3: Torsional fatigue sample designed for this study.

Dimensions in mm.

601

Induction Hardening

Hot rolled specimens were scan induction hardened to

nominal normalized effective case depths (t/r) of 0.25 (low),

0.32 (medium), and 0.44 (high) using a single turn coil on a

100 kW/200 kHz power supply. The low and high case depth

conditions used identical setups with the high case depth

simply having a slower scan speed (17.3 mm/s) than the low

case depth (22.9 mm/s). To achieve the medium case depth, a

pre-heat cycle and quench delay were incorporated into the

setup used for the low and high case depth conditions.

Figure 4 shows transverse macrographs of 10V45 torsional

fatigue specimens at maximum shear stress (minimum

diameter) illustrating low, medium, and high case depths.

Figure 5 shows representative secondary electron SEM images

of the martensitic case microstructure of the 10V45 steel in the

low and high case depth conditions. The high case depth

condition does appear to have a coarser martensitic structure

due to the longer austenitizing time which resulted from the

slower scan speed needed to achieve the high case depth.

(a) (b) (c)

Figure 4: Macrographs of (a) low (25%), (b) medium

(32%), and (c) high (44%) case depths in the

10V45 steel at maximum shear stress location.

(a) (b)

Figure 5: Representative secondary electron SEM images of

the martensitic case microstructure of (a) low case

depth and (b) high case depth conditions in 10V45.

Figure 6 shows spline curve fits for radial microhardness data

for the low (L), medium (M), and high (H) case depth in the

1045 steel at the specimen minimum diameter. The SAE

specification for effective case depth for a 0.45 wt. % C steel

is indicated [18]. All three conditions show similar case peak

hardness as well as core hardness. Figure 7 shows spline curve

fits for radial microhardness data for the low (L), medium

(M), and high (H) case depth in the 10V45 steel at the

specimen minimum diameter. The medium and high case

depths have similar case peak hardness and core hardness

while the low case depth shows significantly higher case peak

hardness as well as possible over aging at the case/core

transition region. The 10V45 conditions showed slightly

higher case hardness as well as core hardness as compared to

the 1045 conditions. The higher core hardness of the 10V45 is

a result of both precipitation strengthening from the vanadium

as well as the higher pearlite fraction and smaller ferrite grain

size. The higher case hardness of the 10V45 is a result of the

slightly higher carbon content in addition to a possible

vanadium effect yet to be determined.

Figure 6: Spline fits for Vickers microhardness traverses at

torsional fatigue specimen minimum diameter

(maximum shear stress location) for 1045 steel.

Figure 7: Spline fits for Vickers microhardness traverses at

torsional fatigue specimen minimum diameter

(maximum shear stress location) for 10V45 steel.

602

Torsional Fatigue

Appropriate stress levels for torsional fatigue testing were

determined by establishing an empirical model from the data

presented in Table 1 and Figure 1. The following multiple

linear regression model was developed

Shear Stress Amplitude (MPa) =

1115.379

– 169.815*log10(Nf)

+ 467.975*(wt. % C)

+ 445.416*(t/r)

(1)

where Nf is the number of cycles to failure between 102 and

3 x 106 cycles, wt. % C is the carbon content of the steel in

wt. % between 0.35 and 0.56, and t/r is the normalized

effective case depth between 0.21 and 0.68. All variables were

found to be significant to the regression above the 99% level.

Standardized residuals as well as the predicted output are

normally distributed. The adjusted coefficient of determination

is 0.8344 for 159 observations used in the model. Figure 8

shows the model graphically with upper and lower 95%

confidence limits for the steels in the current study. The upper

bound was calculated using the highest carbon content and

effective case depth condition, 10V45(H), while the lower

bound was calculated using the lowest carbon content and

effective case depth condition, 1045(L).

Figure 8: Empirical model of torsional fatigue life of

induction hardened medium carbon steels. Shaded

region indicated the upper and lower 95%

confidence limits for the steels and effective case

depths used in the current study.

The model was developed to assist in determining test stresses

that would provide failures between 104 and 106 cycles. As a

result, the following three shear stress amplitudes were

selected: 550, 600, and 650 MPa. Five specimens of each

condition were tested at each shear stress amplitude. A

positive stress ratio of 0.1 was used to preserve the fracture

surfaces since it has been previously shown that stress ratio

has no effect on torsional fatigue performance.

Results and Discussion

Torsional Fatigue Failure Origin

A total of 90 specimens were tested with 79 having a surface

failure origin and 11 having a sub-surface failure origin. Of

the 90 failures only 8 initiated failure outside the 95%

maximum shear stress region, 2 were surface and 6 were sub-

surface initiated. Figure 9 shows example fracture surfaces

exhibiting different failure origins and fracture behavior in

high case depth specimens tested at 550 MPa. Figure 9a shows

Mode I initiation and failure propagation in a 10V45 specimen

while Figure 9b exhibits Mode I initiation followed by

Mode III then Mode I propagation in 1045. The transition in

propagation mode in the 1045 is likely a result of the slightly

lower case hardness of the 1045 in general. Figure 9c shows a

fracture behavior that is indicative of all observed sub-surface

initiated failures. Initiation may be either Mode I or Mode III

followed by Mode I propagation into the case and Mode III

propagation into the core.

Table 3 shows failure origin for all of the torsional fatigue test

as a function of steel and induction hardened case depth. For

the low case depth condition failure predominately originated

at the surface at all stress levels in both the 1045 and 10V45

steels except for 10V45 tested at 550 MPa. In this condition,

4 of 5 specimens failed sub-surface. In the medium case depth

condition all specimens initiated failure at the surface. At the

high case depth both steels exhibited a significant mix of both

surface and sub-surface initiation at the 550 MPa stress level.

(a) (b) (c)

Figure 9: Example torsional fatigue fracture surfaces of

(a) surface initiation in 10V45(H), (b) surface

initiation in 1045(H), and (c) sub-surface initiation

in 10V45(H). All specimens tested at 550 MPa.

Table 3: Number of Surface Initiated Failures by Condition

Shear Stress

Amplitude

(MPa)

1045 10V45

L M H L M H

650 5 5 5 5 5 5

600 5 5 5 4 5 5

550 5 5 2 1 5 2

Torsional Fatigue Stress-Life Behavior

Figures 10, 11, and 12 show 1045 and 10V45 stress-life data

for the low, medium, and high case depth conditions

respectively. Specimens that initiated failure outside the 95%

maximum shear stress region are indicated by a separate

603

symbol on each plot. The 95% confidence limits for the

empirical model are superimposed for each condition for

comparison. The limits were calculated using the applicable

nominal effective case depth along with either 0.43 (lower

limit) or 0.48 wt. % C (upper limit).

In the low case depth condition (Figure 10), a noticeable

difference in fatigue life was observed at all shear stress

amplitudes between the 1045 and 10V45. The 10V45 has

significantly longer fatigue lives that more closely align with

the empirical model. The low case depth 10V45 was

measured to have a higher case hardness than all of the other

conditions tested which may be a reasonable explanation for

its improved fatigue performance.

In the medium case depth condition (Figure 11), fatigue lives

demonstrate very low variability at a given stress level and

appear to be independent of vanadium content. Although this

condition has both deeper effective and total case depths, the

fatigue life is notably worse than the 10V45 low case depth

condition. This result is contrary to literature stating total case

depth is closely related to fatigue performance [19]. This

result suggests the pre-heat cycle and/or the quench delay used

in processing the medium case depth condition may have

influenced factors such as residual stresses which significantly

influence fatigue behavior. In addition, the empirical model

over-predicts the fatigue life of the medium case depth

condition significantly.

In the high case depth condition (Figure 12), very little

difference was observed between the fatigue life of the 1045

and 10V45 steels. At 600 and 650 MPa the fatigue data of the

two steels overlay on each other and the empirical model over-

predicts the fatigue life of both the 1045 and 10V45. At

550 MPa the 1045 steel appears to have increased variability

in fatigue life resulting in the 10V45 having a slightly higher

average fatigue life. Although it is unclear for the 1045, the

empirical model appears to under-predict the fatigue life of the

10V45 steel at 550 MPa in this condition.

Figure 10: Cycles to failure as a function of shear stress

amplitude for the low effective case depth (25%)

condition. Calculated 95% confidence limits from

empirical model included for reference.

Figure 11: Cycles to failure as a function of shear stress

amplitude for the medium effective case depth

(32%) condition. Calculated 95% confidence

limits from empirical model included for reference.

Figure 12: Cycles to failure as a function of shear stress

amplitude for the high effective case depth (44%)

condition. Calculated 95% confidence limits from

empirical model included for reference.

Summary and Conclusions

The torsional fatigue performance of three induction hardened

effective case depths of commercially produced 1045 and

10V45 were compared. Effective case depths of 25, 32, and

44% were produced using a single coil scan-hardening system.

Five specimens per condition were fatigue tested in torsion at

shear stress amplitudes of 550, 600, and 650 MPa. An

empirical model was developed from literature data to

determine test stress levels as well as for comparison. The

most significant observations from the current study are as

follows.

The low case depth 10V45 condition exhibited

1. Significantly higher case hardness than any

other condition.

2. Superior fatigue performance compared to

the 1045 low case depth condition as well as

all medium case depth conditions.

3. Most closely fitting the empirical model of

any condition.

604

The medium case depth condition demonstrated

nearly identical fatigue performance and very low

variability in fatigue data at all stress levels for both

the 1045 and 10V45.

The high case depth condition exhibited very similar

fatigue performance between the 1045 and 10V45 at

all stress levels.

Acknowledgments

Support for this work by the Advanced Steel Processing and

Products Research Center at the Colorado School of Mines,

Inductoheat Inc. of Madison Heights, MI, Fluxtrol Inc. of

Auburn Hills, MI, and Gerdau Specialty Steels North America

is gratefully acknowledged.

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