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Lecture 7

Steels for Surface

Hardening

Dr. Javad Mola

Institute of Iron and Steel Technology (IEST)

Tel: 03731 39 2407

E-mail: mola@iest.tu-freiberg.de

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Applications of Case Hardened Steels

Applications where wear and severe stresses act only on the surface of a part

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Case/Surface Hardening Methods

Layer additions Hardfacing

Fusion hardfacing (welded overlay)

Thermal spray (bonded overlay) Coatings:

Electrochemical plating Chemical vapor deposition

(electroless plating) Thin films (physical vapor

deposition, sputtering, ion plating)

Ion mixing

Substrate treatment Selective hardening methods:

Flame hardening Induction hardening Laser hardening Electron beam hardening Ion implantation Selective carburizing and nitriding Use of arc lamps

Diffusion methods (composition modification at surface):

Carburizing Nitriding Carbonitriding Nitrocarburizing Boriding Titanium-carbon diffusion Toyota diffusion process

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Induction/Flame Hardening

No change in the chemical composition of the surface layer Carbon content must be high enough to achieve a high surface hardness after quenching (typically above 0.35 mass-%C).

Surface hardness is a function of the solute C. Very fast heating and short exposure time necessitates the use of a higher holding

temperature than in through-hardening by normal Q&T High cooling rates possible without cooling medium, only through heat dissipation to

inner sections hardening depth larger than in through-hardening The microstructure at inner sections (tempered martensite in steels with high

hardenability and normalized in unalloyed steels) is not affected by induction/flame hardening

The wear and fatigue resistance of the case-hardened part improves with the increased surface hardness and development of compressive residual stresses

Low temperature tempering/stress relieving usually in the range of 140-200 °C to maintain a high surface hardness

Hardening depth depends on: depth of austenitization cooling rate hardenability of steel

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Superhardness

Causes of higher hardness (superhardness) in case hardened steels:

Suppression of martensite auto-tempering due to a higher cooling rate Smaller austenite grain size due to a shorter exposure time Higher residual compressive stresses at surface

Case-hardened

Through-hardened

Through-hardened

Case-hardened

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Induction Hardening Schematic of the magnetic fields and induced currents

produced by several types of induction coils

Heating is accomplished by placing a steel part in the magnetic field generated by high-frequency alternating current passing through an inductor, usually a water-cooled copper coil. The depth of heating produced by induction is related to the frequency of the alternating current: The higher the frequency, the thinner or more shallow the heating. Therefore, deeper case depths and even through hardening may be produced by lower frequencies.

Outer diameter heating, single-turn coil Outer diameter heating, multi-turn coil

Inner diameter heating, multi-turn coil Surface heating, single-turn coil

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Heating Rate and Starting Microstructure

Change in the Ac3 temperature of 1042 steel as a function of microstructure and heating rate

(1093 °C)

(760 °C)

1042 steel Annealed

Normalized

Q&T

(2778 °C/s) (28 °C/s)

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Induction Hardening: Starting Microstructure

Influence of the microstructure prior to induction hardening on the

hardness profile of SAE 1070 steel with 0.70%C, 0.20%Si, and

0.80%Mn.

Hardness profiles for 1550 and 5150 steels after induction hardening of

two starting microstructures, furnace cooled and water quenched

prior to induction hardening.

Hardening time: 1 sec

Distance from surface, mm Distance from surface, mm

Vic

kers

Ha

rdn

ess

Ha

rdn

ess

(ap

pro

xim

ate

), H

RC

Ha

rdn

ess,

HR

C

Q&T

normalized

annealed

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Flame Hardening

Flame hardening consists of austenitizing the surface of a steel by heating with an oxyacetylene or oxyhydrogen torch and immediately quenching with water. This results in a hard surface layer of martensite over a softer interior core with a ferrite-pearlite structure. There is no change in composition. Therefore, the flame-hardened steel must have adequate carbon content for the desired surface hardness.

Effect of flame speed on depth of hardening of a 1050 steel forging. The slower the rate of travel, the greater the heat penetration and the depth of hardening.

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Flame Hardening Spot (stationary) flame hardening

of a rocker arm

Spot (stationary) flame hardening of the internal lobes of a cam

Progressive flame hardening

Stationary flame head

Rotating workpiece

Rotating workpiece

Stationary flame head

Rotating flame head

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Strength vs Hardening Depth

Torsional strength (major requirement for transmission of torque in shafts and axles) as a function of case depth for various grades of steel

Case Hardening Depth

Radius

Case Hardening Depth

Radius

Tors

ion

al S

tren

gth

, M

Pa

Tors

ion

al S

tren

gth

, M

Pa

Through-hardened Through-hardened

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Carburizing

Carbon content of the surface is increased by exposure to an appropriate atmosphere at a temperature in the austenite phase field. Hardening is accomplished when the high-carbon surface layer is quenched to form martensite.

Steels for carburizing usually have base carbon contents around 0.2%. The maximum carbon content in a carburized steel is generally between 0.8 and 1%. Complications at higher C concentrations:

Carbide formation Brittle martensite Retained austenite

Typical carburizing temperatures: 850-950 °C, but sometimes higher temperatures are used to reduce cycle times and/or produce deeper depths of the high-carbon surface layer.

Two important processes influence the introduction of carbon into austenite: The environmental reaction that causes carbon to be absorbed at the surface of

the steel (adsorption). The rate at which carbon can diffuse from the surface to the interior of the steel

(diffusion). Carbon is introduced by the use of gaseous atmospheres (gas carburizing), salt baths

(liquid carburizing), solid compounds (pack carburizing), and by plasma carburizing.

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Gas Carburizing

Schematic of a gas carburizing process involving partial combustion of a hydrocarbon, which acts as the source of carbon. The CO molecules are

adsorbed and dissociate on the steel surface. The adsorbed C atoms subsequently diffuse into the bulk.

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Gas Carburizing Equilibrium percentages of carbon monoxide and carbon dioxide required to maintain various carbon concentrations at 975 °C in plain carbon and certain low-alloy steels.

Decarburizing

Carburizing

CO content, %

CO

2 c

on

ten

t, %

Gas carburizing can be done using a carburizing hydrocarbon gas (methane, propane, etc.) to supply atomic carbon for adsorption and diffusion.

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Carbon Concentration Profile

Time-dependent diffusion of carbon from the surface to the interior of the

steel part

Solution to Fick’s second law (𝑑𝐶

𝑑𝑡= 𝐷

𝑑2𝐶

𝑑𝑋2)

using the boundary conditions Cc (at X=0)=Cs , and Cc (at x=∞)=C0

Cs: Carbon concentration at surface C0: Initial (bulk) carbon concentration of Fe-C alloy D: Diffusion coefficient for C in austenite (assumed to be independent of chemical composition) X: Distance from surface Cc: Carbon concentration at distance X from surface t: time after the start of carburizing

Fe-C alloy at 925 °C Diffusion coefficient assumed to be

independent of composition.

Cs

X

C0

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Since erf(0.5) 0.5, the depth at which the carbon concentration is midway between C0 and Cs is given by:

𝑿 = 𝑫𝒕 thus the thickness of carburized layer is

approximately 𝑫𝒕 . A two-fold increase of the carburization depth therefore requires a four-fold increase of the carburization time.

Fe-C alloy at 925 °C Diffusion coefficient assumed to be

independent of composition.

Carbon Concentration Profile

Cs

C0

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Oxidation during Carburizing Presence of oxygen in the carburizing atmosphere may lead to internal oxidation in presence of elements such as Si, Cr, and Mn which are commonly found in carburizing. Mo and Ni are not oxidized. Oxidized elements are removed from solid solution in austenite and decrease the hardenability. Early transformation to pearlite and bainite or martensitic transformation at a relatively high temperature will then interfere with the development of compressive stresses at surface. The presence of such intergranular oxidation adversely affects the fatigue resistance.

930 °C

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Intergranular Oxidation during Carburizing

Influence of surface oxidation on the static loading limit and bending fatigue strength of a 15CrNi6 steel after carburizing

Cycles to Failure

Ben

din

g F

ati

gu

e St

ren

gth

, MP

a

Static loading

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Intergranular Oxidation during Carburizing

AISI 8620 (0.2%C, 0.92% Mn, 0.50% Cr, 0.38% Ni, and 0.16% Mo): non-martensitic product formation due to oxidation in carburized 8620 steel with a relatively low hardenability AISI 4615 (0.2%C, 0.52% Mn, 0.12% Cr, 1.75% Ni, and 0.54% Mo): martensite at surface due to the presence of high Mo and Ni concentrations

Fatigue strength Residual stresses

Co

mp

ress

ive

Ten

sile

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Vacuum Carburizing

Vacuum carburizing is a state-of-the-art thermal process where carburizing is done under very low pressures. First the parts are heated in vacuum to above the transformation temperature of the alloy. Then they are exposed to carbon-carrying gas, or gas mixtures, under partial pressure. Relative to conventional carburizing, the main advantages of the method are better control of the surface layer chemistry and thereby improved fatigue strength.

Intergranular oxidation

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Alloy Carbide Formation

Effect of Cr content on the carbon content of steels after carburizing. In presence of carbide forming elements such as Cr, some of the carbon introduced by carburizing may form alloy carbides.

Chromium Content, mass-%

Ca

rbo

n C

on

ten

t a

t Su

rfa

ce, m

ass

-% Carburizing conditions

Pu

re ir

on

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Residual Stresses

Schematic diagram of residual stresses in carburized steels; surface

compressive stresses are balanced by interior tensile stresses.

Measured and estimated residual stresses as a function of distance through the

thickness of carburized and uncarburized steel specimens

L

R

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Residual Stresses

Shot peening can increase the surface residual stresses developed by carburizing of 16MnCr 5 steel (1.23%Mn, 1.08%Cr) and improve its fatigue strength.

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Retained Austenite after Carburizing

Influence of alloying elements in solid solution on the martensite start temperature and the fraction of retained austenite in Fe-1%C steel.

Fe-1%C Fe-1%C

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Fatigue Strength

The effect of reheating after carburizing, on the fatigue resistance of a carburized coarse-grained 8620 steel. All specimens were chemically polished. Therefore, the influence of intergranular oxidation on fatigue cracking was removed from the study.

Coarse structure, high density of microcracks

Lower density of surface microcracks, finer martensitic structure, higher hardness, and reduced retained austenite fraction

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Fatigue Strength

The effect of austenite grain size and retained austenite content, controlled by reheating after carburizing, on the fatigue strength of 8719 steel. Finer austenite grain size and reduced fraction of retained austenite achieved by reheating to lower austenitization temperatures after carburizing contribute to the fatigue strength.

Gas carburizing

Direct quench

Gas carburizing

Single reheat

Plasma carburizing

Direct quench

Plasma carburizing

Single reheat

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Fatigue Strength

Fatigue limit as a function of austenite grain size of 8719 steel carburized and

hardened as indicated

Fatigue limit as a function of retained austenite content in

8719 steel carburized and hardened as indicated

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Fatigue Strength

Fatigue limit as a function of austenite grain size and retained austenite content of carburized 8719 steel

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The role of retained austenite in the carburized layer on the fatigue strength (positive or negative) depends on factors such as the chemical composition of retained austenite, type of loading, and pre-stressing.

Cycles to Failure

Stre

ss, M

Pa

Retained no tempering

Retained

No retained

(601 HRC)

Retained , rolled with a pressing force of 1400 N

Scatter band for retained , shot peened

Scatter band for no retained , shot peened

(601 HRC)

Fatigue Strength

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Reheating

Retained austenite, measured by X-ray diffraction, as a function of distance from the surface of carburized 8620 steel. Reduction of retained austenite takes place by reheating below Acm temperature where the formation of undissolved carbides increases the martensite start temperature.

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Reheating Hardness profile near surface of a 16MnCr 5 steel after salt bath carburizing

at different temperatures with or without a single reheat at 825 °C.

Carburizing temperature: Distance from Surface, mm

Carburizing Time V

icke

rs H

ard

nes

s, H

V0

.5 Direct Quench

Single Reheat

Base (core) hardness

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Nitriding

Gas nitriding is a case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature below (Ac1) in contact with a nitrogenous gas, usually ammonia. Quenching is not required for the production of a hard case. The nitriding temperature for all steels is between 495 °C and 565 °C. Principal reasons for nitriding are:

To obtain a high surface hardness To improve:

• wear resistance and anti-galling properties • fatigue life • corrosion resistance (except for stainless steels)

To obtain a surface that is resistant to the softening effect of heat at temperatures up to the nitriding temperature

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Nitriding Sequence

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Steels for Nitriding

Of the alloying elements commonly used in commercial steels, Al, Cr, V, W, and Mo are beneficial in nitriding because they form nitrides that are stable at nitriding temperatures. Mo, in addition to its contribution as a nitride former, also reduces the risk of embrittlement at nitriding temperatures. Other alloying elements, such as Ni, Cu, Si, and Mn have little effect on nitriding characteristics. Although at suitable temperatures all steels are capable of forming iron nitrides in the presence of nascent (atomic) nitrogen, the nitriding results are more favorable in those steels that contain one or more of the major nitride-forming alloying elements. Because Al is the strongest nitride former of the common alloying elements, Al-containing steels (0.85 to 1.50% Al) yield the best nitriding results. Cr-containing steels can approximate these results if the Cr content is high enough. Unalloyed carbon steels are not well suited for gas nitriding because they form an extremely brittle case that spalls readily, and the hardness increase in the diffusion zone is small.

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Iron Nitrides

Iron carbides (binary Fe-N system)

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Iron Nitrides

Ammonia concentrations in ammonia-hydrogen mixtures and temperature ranges

for the formation of various Fe-N phases.

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Alloy Nitrides

Influence of some important alloying elements on the nitriding response (surface hardness after gas nitriding) of steel. Presence of nitride-forming elements increases hardness after nitriding.

Alloying Element Content, mass-% Cr Content, mass-%

Bri

nel

l Ha

rdn

ess

Vic

kers

Ha

rdn

ess

Steel with 1%Al

CrMoV steels

Cr(Mo) steels

Nitriding at 500 °C for 84 hrs (in NH3)

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Carbonitriding

A modified form of gas carburizing rather than a form of nitriding A mixture of (ammonia + carburizing atmosphere) Typically done at a lower temperature and for a shorter time than gas carburizing,

producing a shallower case than is usual in carburizing Hard, wear-resistant case, generally from 0.075 to 0.75 mm deep Better hardenability than a carburized case (nitrogen increases the hardenability of

steel); a hardened case can be produced at a lower cost. Full hardness with less distortion can be achieved with oil quenching, or, in some

instances, even gas quenching, employing a protective atmosphere as the quenching medium.

Commonly applied to steels containing up to about 0.25 mass-%C. Many steels with a carbon range of 0.30 to 0.50% are carbonitrided to case depths

up to about 0.3 mm when a combination of a reasonably tough, through-hardened core and a hard, long-wearing surface is required (shafts and transmission gears are typical examples).

Applied at 845 °C to such steels as 4140, 5130, 5140, 8640, and 4340 for applications like heavy-duty gearing.

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Carbonitriding

One major advantage of carbonitriding is that the nitrogen absorbed during processing lowers the critical cooling rate of the steel. That is, the hardenability of the case is significantly greater when nitrogen is added by carbonitriding than when the same steel is only carburized. This permits the use of steels on which uniform case hardness ordinarily could not be obtained if they were only carburized and quenched. Where core properties are not important, carbonitriding permits the use of low-carbon steels, which cost less and may have better machinability or formability.

End-quench hardenability curve for 1020 steel carbonitrided at 900 °C (1650 °F) compared with curve for the same steel carburized at 925 °C (1700 °F). Hardness was measured along the surface of the as-quenched hardenability specimen. Ammonia and methane contents of the inlet carbonitriding atmosphere were 5%; balance: carrier gas.

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Carbonitriding

The amount of retained austenite was reduced by: - lowering the ammonia flow rate/content of the atmosphere - introducing a 15-min diffusion period at the end of the carbonitriding operation.

Hardness profiles in a 1117 steel carbonitrided at 815 °C for 1.5 hrs and quenched in oil

High retained

austenite content

at surface

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Diffusion Treatment Characteristics Process Nature of case Process

temperature, °C Typical case depth

Case hardness, HRC

Typical base metals Process characteristics

Carburizing Pack Diffused carbon 815-1090 125 µm-1.5

mm 50-63 Low C steels, low-C

alloy steels Low equipment costs, difficult to control case depth accurately

Gas Diffused carbon 815-980 75 µm-1.5 mm 50-63 Low C steels, low-C alloy steels

Good control of case depth, suitable for continuous operation, good gas controls required, can be dangerous

Liquid Diffused carbon and possibly nitrogen

815-980 50 µm-1.5 mm 50-65 Low C steels, low-C alloy steels

Faster than pack and gas processes, can pose salt disposal problem, salt baths require frequent maintenance

Vacuum Diffused carbon 850-1090 75 µm-1.5 mm 50-63 Low C steels, low-C alloy steels

Excellent process control, bright parts, faster than gas carburizing, high equipment costs

Nitriding Gas Diffused

nitrogen, nitrogen compounds

510-565 125 µm-0.75 mm

50-70 Alloy steels, nitriding steels, stainless steels

Hardest cases from nitriding steels, quenching not required, low distortion, process is slow, is usually a batch process

Salt Diffused nitrogen, nitrogen compounds

510-565 2.5 µm-0.75 mm

50-70 Most ferrous metals including cast irons

Usually used for thin hard cases below 25 µm, no white layer, most are proprietary processes

Ion Diffused nitrogen, nitrogen compounds

340-565 75 µm-0.75 mm

50-70 Alloy steels, nitriding steels, stainless steels

Faster than gas nitriding, no white layer, high equipment costs, close case control

Carbo-nitriding Gas Diffused carbon

and nitrogen 760-870 75 µm-0.75

mm 50-65 Low C steels, low-C

alloy steels, stainless steels

Lower temperatures than carburizing (less distortion), slightly harder case than carburizing, gas control critical

Liquid (cyaniding)

Diffused carbon and nitrogen

760-870 2.5-125 µm 50-65 Low C steels Good for thin cases on noncritical parts, batch process, salt disposal problems

Ferritic nitro-carburizing

Diffused carbon and nitrogen

480-590 2.5-25 µm 40-60 Low C steels Low-distortion process for thin case on low-carbon steel, most processes are proprietary

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