AMM-2017-4.indd

Arch. Metall. Mater. 62 (2017), 4, 2247-2254

DOI: 10.1515/amm-2017-0331

P. SKUBISZ*

, A. ŁUKASZEK-SOŁEK*

EFFECT OF PROCESSING CONDITIONS ON FORGEABILITY AND PROPERTIES OF

HOT AND WARM-FORGED STEEL 300M

Results of investigation of the effect of processing conditions of medium-carbon alloy steel AISI 300M on forgeability and

microstructure-properties are presented here, including as-forged and heat treated condition of the material. The presented results

concern two vital aspects of plastic forming of high-duty impression-die forgings, which underlay a selection of technological

conditions which enable the accomplishment of the required quality of the forged part. These are: firstly, the microstructure and

mechanical properties and their uniformity within a part and secondly, technological realization of forging the required geometry

in the given processing conditions.

In order to define a favourable processing window, dynamic behaviour modeling in variable forging conditions was carried

out, establishing a coefficient of energy dissipation η% and the metal flow instability areas, which indicated the temperature regime

and strain rate range for the forging process. The constructed processing maps were subject to experimental verification in the

die-forging tests, carried out on a screw press. Hot and warm forging conditions were applied accordingly to selected areas of the

processing maps, representative for unique forging conditions occurring in the industrial practice.

Keywords: warm forging, processing maps, steel 300M, forgeability, dynamic material modeling

1. Introduction

Steel 300M is an advanced grade of medium carbon ultra-

high strength steel referred to as ultra-high strength steel, whose

composition, besides superior strength properties, is designed to

amend for insufficient level of some operational characteristic,

e.g. fracture toughness and/or cracking resistance. Owing to

high strength, 300M steel is typically used in applications where

superior strength properties are required, e.g. aircraft landing

gears, airframe parts, fasteners, gears, transmission system

components, pressure vessels etc [1,2].

Significant fraction of the applications are complex geom-

etry parts, manufactured through forging techniques. Like other

high strength steels, 300M is typically hot-forged [3]. Hot forging

does not enable high accuracies to be achieved without coining,

strengthening or calibration. Due to its chemistry, this steel is

prone to decarburization while soaking at elevated temperatures.

Therefore, machining was used after normalizing annealing to

attain the final properties.

In many cases, accuracy and quality problems witnessed in

hot forging can be reduced by the application of warm forging

regime. As a rule, forging in lower working range is advantageous

from the standpoint of net-forging accuracies, promoting material

and machining savings, and due to other aspects such as prevent-

ing scale formation, excessive grain growth and decarburization

to which grade 300M has a particular inclination [4]. However,

successful use of warm forging is conditioned by two major

aspects – i) forgeability, and ii) the final microstructure which

in warm forging gain more importance. Both of these aspects

can be taken into account in dynamic material modelling for the

design of the forging process with use of processing maps, which

combines macroscopic studies of flow stress dependence on the

process conditions with microstructural response, referring to

dynamic behaviour of deformed material and stability of plastic

deformation [5-7]. The presented study concerns two vital aspects

of plastic forming of high-duty impression-die forgings, which

underlay selection of the technological conditions which enable

accomplishment of the required quality of the forged part. These

are: firstly, the microstructure and mechanical properties and their

uniformity within a part, and secondly, technological realization of

forging the required geometry in the given processing conditions.

2. Experimental

2.1. Material characterization

As-received steel 300M in the form of a hot-rolled bar was

used in the study. The chemical composition of the grade used in

the study is presented in Tab. 1. The experiment was preceded

* AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY IN CRACOW, DEPARTMENT OF METALS ENGINEERING AND INDUSTRIAL COMPUTERS SCIENCE, AL. MICKIEWICZA 30, 30-059

KRAKÓW, POLAND

Corresponding author: [email protected]

2248

by as-received metallographic characterization and dilatometric

examination in order to estimate initial microstructure and char-

acteristic points of transformation, which were used for definition

of forging temperatures. As indicated in volumetric heat expan-

sion tests carried out for the same heat [8], austenite formation

began at 770°C (A

), and ferrite-austenite transformation com-

pleted in 850°C (A

). The initial microstructure (as shown in

Fig. 1) in as-received condition consisted of bainite-martensite

mixture with fine particles identified as cementite (Fig. 1c).

TABLE 1

Chemical composition of steel 300M used in the study

Alloying

element

CMnSiCrMoNi S P VFe

Content,

wg. %

0,38 0,6 1,45 0,7 0,29 1,65 0,01 0,01 0,05 bal.

2.2. Flow behaviour analysis

Flow curves were elaborated for the needs of description

of the rheological behaviour of steel 300M in numerical mod-

eling with finite element method (FEM) and construction of

the processing maps. The flow curves (set together on graphs

in Fig. 2.) were derived from compression tests conducted on

testing machine Gleeble 3800. FEM modeling of the upset

forging process was carried out with code QForm3D with as-

sumption of rigid-plastic model of deformed body and Levanov

friction model, applying coupled thermal-mechanical analysis

for solution of actual temperature, strain and stresses fields in

the points of interest.

2.3. Dynamic behaviour modeling

Hot deformation characteristics were established based on

uniaxial compression tests. Compression tests were carried out

in a wide range of temperatures (800-1200°C) and strain rates

(0,01-100 s

–1

). Obtained stress-strain curves were a basis for

correlation between Zener-Hollomon parameter and flow stress,

for which the constitutive equation proposed by Sellars was

used. Having estimatied of activation energy (Q

śr

= kJ· mol

–1

coefficient of energy dissipation η % and processing maps

were constructed. The maps indicated four windows of the

metal flow instability. As an experimental verification of the

theoretical analysis of the flow behaviour, experimental tests

Fig. 1. Characterization of the material used in the study: a-b) microstructure in as-received condition, c) identification of the precipitates with

x-ray diffraction

2249

were conducted with screw press and hammer forging in lower

temperature range of the work regime, 800-850°C. Obtained

samples exhibited metallurgical soundness, however, indicated

nonuniformity of deformation reflected by variations microstruc-

ture resultant from selective deformation and inhomogeneous

pattern of recrystallized grains. As suggested by the calculated

processing windows, increasing the work temperature resulted

in improvement in microstructure uniformity, reflected by me-

chanical properties in as-forged condition.

Workability analysis carried out for selection of the most

favourable forging conditions for steel 300M was based on

Dynamic Material Modelling method [5-7,9,10] with the use of

Prasad approach [5,6,11-13]. In the Dynamic Material Model

(DMM) the unit power (P) absorbed – by the material during

plastic working is expressed in the following way [5-7,9,10]:

PGJdd

VH V H HV

  

³³





(7)

where G – dissipator content (represents the power dissipated

by plastic work, which is converted into heat), J – dissipator co-

content, a component which represents power dissipation into

microstructural transformations, such as dynamic recrystalliza-

tion, dynamic recovery and/or grain growth. The value of this

component can be calculated from the formula:







(8)

where: ε constant true strain value, σ – the flow stress, MPa,

the strain rate, s

–1

, m parameter of the strain rate sensitivity of

flow stress, usually referred to as

log

§·

¨¸

©¹



(9)

The efficiency of power dissipation (η), as a measure of the

ability to undergo plastic deformation, was estimated from the

equation proposed by Prasad

max



(10)

where J : dissipator co-content

max







, – or m = 1,

m – coefficient describing sensitivity to strain rate [5,6]. With

the use of calculated value of η(%), maps of dissipation power

efficiency were built. The criterion for identification of the

metal flow instability during hot deformation put forward by

Ziegler [14] is



log

§·

¨¸



©¹





(11)

With the assumption that the parameter ξ ≤ 0 is reached,

microstructural instability of the metal flow is concluded,

reflected by occurrence of e.g. adiabatic shearing bands, strain-

induced dynamic ageing, flow turbulences. Process instability

is a complex notion referring to a given temperature and strain

rate, which can be referred to as the conditions under which the

deformation takes place. Parameter ξ forms a sort of warning

while designing the process for a new material, whereas any

changes in the value of the parameter enable the construction

of an instability map.

The processing mp is made up by superposition of power

dissipation effectiveness map (η(%) elucidated with isoclines)

with the metal flow instability parameter (ξ)).

2.2. Forging test conditions

Physical verification was carried out by means of physical

modelling. The experimental results were to confirm the appli-

cability of the processing maps in design of the forging process

in aspects of producing sound part and proper microstructure,

Fig. 2. Stress-strain characteristics derived from uni-axial compression tests on Gleeble 3800 for: a) constant strain rate 10 1/s, b) constant tem-

perature 1100°C

2250

which is indirectly indicated by instability and energy dissipation

parameters, respectively. For assessment of final properties of

the as-forged steel 300M microstructure was analysed and tensile

properties established.

The forging tests were conducted with screw press and

hammer forging in hot forging (1000°C) and within the range

of the warm work temperature range, 800-850°C.

The rolled bar was cut on sections of length resultant

from the volume the forged part including flash and subject to

homogenization annealing in accordance with temperature re-

gime common for that grade [3] and heated up/cooled down to

forge temperature. Deformation took place after soaking at the

deformation temperature for 20 minutes. Forging experiment was

carried out on a screw press of maximum flywheel energy 16 kJ

and ram starting speed 0.7 m/s. Based on dilatometric curves

characterized for this heat [8] transformation points were deter-

mined, which allowed selection of forging temperatures. Three

different deformation temperatures were used, so as to represent

three unique temperature ranges: 1) 1000°C – hot-work range,

2) 850°C – lower range of hot-forging regime, and 3) 800°C –

intercritical range between A

and A

, as shown in Fig. 3a)

Pyrometer measurement was maintained for tracing the

actual forging temperature and calibrated with established emis-

sivity characteristics to ensure accuracy [15] as well as thermo-

couple measurement during soaking, forging and cooling, with

probe located in the core of the billet at the half of the height.

From the forged samples (Fig. 3b) specimens for metallographic

work and tensile testing were extracted, in direction suggested

by ultrasonic detection so as to omit post-probe discontinuities.

Fig. 3. The experimental specimens: a) plan of the tests, b) the forged

samples

3. Results

3.1. Workability estimation with processing maps

In assessment of warm-range forgeability of steel 300M,

significant sensitivity of the power dissipation and stability of

deformation in temperature 800÷900°C can be concluded. It

means that there is a linear relation between instant values of

the strain rate hardening exponent and temperature increment.

The processing maps constructed for the Cr-Mo steel 300M

(Fig. 4) indicate the areas of metal flow instability (dark fields),

for which ξ ≤ 0, and elucidated with isoclines levels of power

dissipation, η%. For the considered range of strain ε = 0.1, 0.3,

0.5, 0.7 and 0.9 equivalent domains (processing windows of

strain rate promoting highest efficiency of plastic deformation

at temperature concerned) can be determined. They are the basis

for definition of suggested values of these parameter for forging

process, which for warm forging ranges between ε

= 1÷10 s

–1

and for hot forging ε

= 0.03÷1 s

–1

, providing driving force for

dynamic recovery and recrystallization with satisfactory stability

of the deformation process, without local energy accumulation.

According to thus constructed maps, proper stability of the

forging process conditions is expected within 825÷900°C and

1000÷1170°C. For strain degree 0.9 two processing windows

can be distinguished: rectangular field featuring efficiency

η = 19-24% for temperature 825÷900°C and strain rate ranging

1÷10 s

–1

. The available window corresponds to forging small or

flat geometries on screw or hydraulic press.

Isoclines of parameter η% attain density and gradient con-

forming to increasing temperature, and their contours suggest

uniformity and the metal flow stability during forging stages.

The other processing window found for temperatures

1000÷1170°C and strain rate 0.03÷1 s

–1

is contained in the range

of isoclines efficiency η = 22-32%. The isoclines of the power

dissipation parameter feature a slight increments from eachother

occasionally bulging towards higher speeds at 1000÷1100°C

switching to higher density within 1100÷1170°C. It may mean

that the plastic flow resistance is function of ε, ε

, T, and the

behaviour is representative of the response of the material hot

forged on slow action hydraulic presses.

When reaching 0.9 two areas of flow instability can be

found. The first one is located within the range 800-825°C above

the strain rate of 56 s

–1

. Adiabatic shear bands and/or microstruc-

tural defects may occur in that area. The coordinates of the other

one are 800÷875°C and 0.01-0.25 s

–1

. As far as the intermediate

amount of strain, ε = 0.1-0.7 is concerned, the areas extend and

move towards lower Zener-Hollomon values.

3.2. Properties of the forged samples

Hot forged specimens after air cooling produced typical

microstructure for both of the analysed regions. Beneath 40 μm

deoxitated case of ferrite, the undersurface regions grade 300M

exhibits martensitic-bainitic structure with the laths reaching

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Fig. 4. Processing maps in function of temperature and strain rate with consideration of instability areas constructed for steel 300M with Prasad

approach for true strain: a) 0.1, b) 0.3, c) 0.5, d) 0.7, e) 0.9

2252

12 μm in length, which come down to average 6-7 μm in the

bulk. In the warm forged specimens, the grains are outlined with

proeutectoid ferrite which at lower temperature appears mixed

with pearlitic and/or bainitic colonies and features subgrain for-

mation in the ferrite phase. In the interior of the bulk specimens

the microstructure consists of pearlitic-ferritic microstructure of

recrystallized grains of 9 ASTM grain size (Fig. 5b).

It is obvious that the phase composition is related to hard-

enability and location concerned, whereas the grain structure

can tell more about dynamic behaviour. In both cases the grain

structure is similar, although no distinct flow localization was

observed, the grain size was not uniformly distributed, in the

(Fig. 5b,c,e,f) areas of 9 ASTM in vicinity of 6-7 ASTM can be

seen. This tendency is more distinct in the intermediate forging

regime, where preferential flow localization in the unstable

region of austenitic transformation took place in the aftermath

of heat generation. In the both forging ranges, hot and warm,

the forged material indicated relatively similar flow behaviour,

resulting in sound forgings, wherein no separation or cracking

occurred. In many studies concerning workability assessment

with processing maps, the microstructure is quenched practiced

so as to freeze the as-forged condition. In this work the idea was

Fig. 5. Microstructure of analysed steel 300M forged at: a), d) 800°C; b), e) 850°C and c), f) 1000°C in the undersurface region (a-c) and in the

axis (d-f)

2253

to reflect the material behaviour in a state typical of air-cooled

condition, representative of good hardenability steels. Thus

the workability and microstructure evolution can be evaluated

through the obtained mechanical properties, shown in Table 2.

From the mechanical properties and microstructure one can con-

clude the workability was satisfactory in the light of providing

amount of deformation required for high strength and fair ductil-

ity, typical of quenched condition, in reference to related studies

[16,17]. The microstructure is even throughout the cross-section

and the final features depend rather on the cooling conditions

and related to amount of martensite and rest austenite, uniform-

ity of strength properties can be assumed from the standpoint of

former austenite grain size after forging [18].

TABLE 2

Tensile properties of steel 300M in as-forged condition

Forging,

tempera-

ture, [°C]

TYS,

MPa

UTS,

MPa

Elon-

gation,

Area re-

duction,

V-notch

Impact

strength,

J/cm

Hardness

HRC

1000 1505 2231 5,8 22 43,3 57

850 1143 1885 6,6 21 34,0 55

800 855 1634 7,8 12 33,2 53

Numerically estimated values of strain and strain rate

(Fig. 5f – grey line) observed in the axis are linked with maps in

(Fig. 5c-d). The processing window locates between two instabil-

ity areas. From the standpoint of workability that provides are

relatively safe processing conditions, however, there is a local

minimum of energy dissipation found for strain rate ranging

from 10

to 10

–1

, which means that under these conditions the

material is at closest to viscosity. Bigger failure hazard could be

expected in the location which lies in the near-surface region of

hexagonal face (Point 2 – black line in Fig. 5f), where strain rate

reaches double values or in the flash area, where it grows up to

90 s

–1

. However, the high strain stage of forging was preceded

by considerable deformation at moderate strain rate, enhancing

plasticity. The corresponding region on high-strain processing

maps (Fig. 5d-e) is closer to the “upper” instability area, which

here is shifted to lower temperature range and the area of low

energy dissipation forms a wedge reaching 1000°C. Yet the hot

forging was still beyond this area, resulting in austenite grain

size comparable to those observed in the bulk.

Confronting the resulted microstructure and properties with

processing maps, it can be said the material demonstrates rela-

tively good forgeability, irrespective of the temperature regime.

Comparing the material condition between analysed locations

(Fig. 5a-c) versus (Fig. 5d-f), much lesser difference is observed

than could be expected from the values of strain and strain-rate

concentration in the axis of the specimens (Point 1 in Fig. 4f)

and the microstructure in the surface (Point 2 in Fig. 4f) and,

respectively, isoclines at corresponding strains. The surface of

the specimens was good, without signs of separation or rupture.

One may conclude that the workability of steel 300M at lower

hot forging temperature and in intercritical region is relatively

good. As indicated in strain rate and temperature plots derived

from numerical simulation, screw press, similarly to mechanical

press, offers rates of straining which locate just between two in-

stability regions which may be ascribed to hammers on one side

and hydraulic presses on the opposite, where a large instability

region persists from small to large strain levels. In addition to

increased strain rates, the area is omitted due to increasing actual

temperature by deformation heat generation.

5. Conclusions

The presented study allowed the analysis of the flow be-

haviour of ultra-high strength steel 300M with use of dynamic

material modelling and elaboration of processing maps. The

main conclusions it allowed to formulate are:

1. Definition of sensitive areas of energy dissipation into solid

state dynamic transformations and regions of metal flow

instability during forging indicated „safe” forging regime

of temperature and selection of equipment for realization

in selected temperature range. In this respect screw and

mechanical presses provide suitable strain rate for average

upset-forged geometry.

2. Workability indices defined by instability coefficient and

energy dissipation coefficient in the function of strain rate

versus working temperature suggest relatively good work-

ability of the material both in hot and warm forging tem-

perature. The processing window in which observed strain

rates, 20÷90 s

–1

are found, exhibits maximum viscosity up

to 1000°C temperature, which means the least energy is

dissipated into dynamic phenomena which could contribute

to the metal flow instability. Thus, warm forging regime

offers good technological conditions for forging on a fast

action press, preventing from detrimental surface effects,

such as excessive scale formation or decarbonization.

Acknowledgements

Financial assistance of MNiSzW within the statutory funds in the frame -

work of agreement 11.11.110.292 is acknowledged.

Special thanks are expressed to Krzysztof Kłaput of forge plant Śrubena

Unia S.A. for facilitating the industrial forging equipment and to Tadeusz

Skowronek, Piotr Bała, Marek Paćko and Joanna Kowalska for assistance

in material characterization.

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