Chapter 1 & 3Resuming basic principles ofmetallurgy and construction

special steelsFabrizio D’Errico

Politecnico di Milano Dept. Mech. Engineering

Atomic arrangements of metals

Figure 1.5 - Schematic diagrams of the various stages in the solidification of apolycrystalline material; the square grids depict unit cells. (a) Small crystallite nuclei. (b)Growth of the crystallites; the obstruction of some grains that are adjacent to oneanother is also shown. (c) Upon completion of solidification, grains having irregularshapes have formed. (d) The grain structure as it would appear under the microscope;dark lines are the grain boundaries.

Real configuration of a unit cell

Fig.1.7 - For the face centered cubic crystal structure, (a) a “hard sphere” unit cell representation, (b) a reduced-sphere unit cell, and (c) an aggregate of many atoms.

Interstitial sites

Figure 1.8 - The location of the interstitial sites in cubic unit cells. Only representative sites are shown. The name of the site (e.g. octahedral, tetrahedral, etc.) depends on its location in the cell.

Figure 1.9 - The location of the octahedral site in a face centered cubic unit cell.

Imperfections in the Atomic and Ionic Arrangements: Point defect

Figure 1.10 - Two-dimensional representations of a vacancy and a self-interstitial.

Figure 1.11 - Point defects: (a) vacancy, (b) interstitial atom, (c) small substitutional atom,(d) large substitutional atom.

Interfacial defects: grain boundary

Figure 1.14 – Schematic diagram showing small and high-angle grain boundaries and the adjacent atom positions.

Figure 1.16 Schematic diagram showing atwin plane or boundary and the adjacentatom positions (colored circles).

Mechanisms of diffusion into metal

Figure 1.17 – Schematic representations of (a) vacancy diffusion and(b) interstitial diffusion.

Line defects: dislocations in metal

Figure 1. 21 – Constructions of a line defect (dislocation): (a) a perfect crystal; (b) an extra half-plane of atoms is inserted; (c) the bottom edge of the extra plane is an edge dislocation.

Figure 1.22 - The atom positions around an edge dislocation; extra half-plane of atoms shown in perspective

Dislocation movement: atomic scale

Figure 1.25 - Line dislocation movement

No force applied, dislocation inside grain do not move

Force applied, maximum shear stress induces dislocations on 45°

slip planes to move to grain boundary

45°

σ

σ

Figure 1.26 – Permanent deformation occurring at grain boundaries in polycrystalline metals that reveal macroscopic deformation onto surface of specimen under tensile loading.

Dislocation movement: microscopicscale

Microstructures of deformed grains

Figure 1.32 - Slip lines on the surface of a polycrystalline specimen of copper that was polished and subsequently deformed.

Figure 1.33 - Alteration of the grain structure of a polycrystalline metal as a result of plastic deformation. (a) Before deformation the grains are equiaxed. (b) The deformation has produced elongated grains.

Strengthening mechanisms in metals

- Solid solution (metal alloying)

- Second phase precipitation (dispersion strengthening)

- Work hardening (dislocation interlocking )

- Grain refinement:

the HALL-PETCH correlation 𝜎𝜎𝑦𝑦= 𝜎𝜎0 + 𝐾𝐾𝑦𝑦 � 𝑑𝑑−12

Softening in metals

- Recovery

- Grain growth

- Recrystallizazion

DEVELOPMENT OF MICROSTRUCTURE IN IRON–CARBON ALLOYSThe PERLITE structure in euctectoidic systems

Fig. 22 – Schematic representations of themicrostructures for an iron–carbon alloy of eutectoidcomposition (0.76 wt% C) above and below theeutectoid temperature.

Fig. 23 – Photomicrograph of aeutectoid steel showing thepearlite microstructureconsisting of alternating layersof -ferrite (the light phase) andFe3C (thin layers, most ofwhich appear dark).

Fig. 24- Schematicrepresentation of theformation of pearlite fromaustenite; the direction ofcarbon diffusion isindicated by arrows.

DEVELOPMENT OF MICROSTRUCTURE IN IRON–CARBON ALLOYSThe PERLITE-FERRITE structure in Hypo-Eutectoid systems

Fig.25 - Schematic representations of the microstructures for an iron–carbon alloy of hypoeutectoid composition C0 (containing less than 0.76 wt% C) as it is cooled from within the austenite phase region to below the eutectoid temperature.

Fig.26 - Photomicrograph of a 0.38-wt% C steelhaving a microstructure consisting of pearliteand pro-eutectoid ferrite (i.e. ferrite grains formed above euctoidic temperature)

DEVELOPMENT OF MICROSTRUCTURE IN IRON–CARBON ALLOYSThe PERLITE-CEMENTITE structure in Hyper-Eutectoid systems

Fig.27 - Schematic representations of the microstructures foran iron–carbon alloy of hypereutectoid composition C1(containing between 0.76 and 2.14 wt% C) as it is cooledfrom within the austenite-phase region to below theeutectoid temperature.

Fig.28 - Photomicrograph of a 1.4-wt% C steelhaving a microstructure consisting of a whiteproeutectoid cementite network surrounding thepearlite colonies..

A diagram showing the relationship between carbon content, microstructure and mechanical properties of EQUILIBRIUM STRUCTURES (Ferrite , Perlite , Cementite) in plain carbon steels

Phase Transformations in STEELS by heat treating

Use of TTT diagramAn actual isothermal heat treatment curve (ABCD) is superimposed on theisothermal transformation diagram for a eutectoid iron–carbon alloy inFig.31.

NOTE: Very rapid cooling of austenite to a given temperature is indicated by the near-verticalline AB, and the isothermal treatment at this temperature is represented by the horizontalsegment BCD.

Fig.31 - Isothermal transformation diagram fora eutectoid iron–carbon alloy, withsuperimposed isothermal heat treatmentcurve (ABCD). Microstructures before, during,and after the austenite-to-pearlitetransformation are shown.

The Bainite structure in steels• In addition to pearlite, other microconstituents that are products of the austenitic transformation

exist; one of these is called bainite.• The microstructure of bainite consists of ferrite and cementite phases, and thus diffusional

processes are involved in its formation.• Bainite forms as needles or plates, depending on the temperature of the transformation

(needles resolution is possible only using electron microscopy), Fig.35.

Fig.35 - Bainite shows elongated and needle-shaped particles of Fe3C within a ferrite matrix. Fig.36 - Isothermal transformation diagram for

an iron–carbon alloy of eutectoid composition,including austenite-to-pearlite (A–P) andaustenite-to-bainite (A–B) transformations.

Non-equilibrium structure Martensite• Another microconstituent or phase called martensite is formed when austenitized iron–

carbon alloys are rapidly cooled (or quenched) to a relatively low temperature (in the vicinityof the ambient).

• Martensite is a non-equilibrium single-phase structure that results from a diffusionlesstransformation of austenite.

• The martensitic transformation occurs when the quenching rate is rapid enough to preventcarbon diffusion. This occurs in such a way that the FCC austenite experiences apolymorphic transformation to a body-centered tetragonal (BCT) martensite.

• A unit cell of this crystal structure (Figure 11.21) is simply a body-centered cube that hasbeen elongated along one of its dimensions; this structure is distinctly different from that forBCC ferrite

Fig.38 - The bodycentered tetragonal unit cell (BCT) for martensitic steelshowing iron atoms (circles) and sites that may be occupied by carbon atoms (xs).

Fig.39 – Photomicrograph showing the martensitic microstructure.

Key topics:

Carbon content and quenching mean influences amount of martensite (fully quenched martensite) in steels

The more is carbon content in plain steel, the more is BCT distortion, the more is resistance and brittleness of martensite

Tempering of quenched martensite

0.5% C steel: water quenched and tempered for 48h at 650°C

CONTINUOUS-COOLING TRANSFORMATION DIAGRAMSThe euctectoidic steels

Fig.43 - Moderately rapid and slow cooling curves superimposed on a continuous-cooling transformation diagram for a eutectoid iron–carbon alloy.

• Bainite will not form when an alloy of eutectoid composition or, for that matter, any plain carbon steel is continuously cooled to room temperature (all of the austenite will have transformed into pearlite by the time the bainite transformation has become possible).

• For this reason, the region representing the austenite–pearlite transformation terminates just below the nose (Figure 43), as indicated by the curve AB.

• For any cooling curve passing through AB, the transformation ceases at the point of intersection.

• With continued cooling, the unreacted austenite begins transforming into martensite upon crossing the M(start) line.

Critical cooling rate

• For the continuous cooling of a steel alloy, there exists a critical quenching rate, which represents the minimum rate of quenching that will produce a totally martensitic structure.

• The critical cooling rate curve, when included on the continuous transformation diagram, will just miss the nose at which the pearlite transformation begins

Fig.44 - Continuous-cooling transformationdiagram for a eutectoid iron–carbon alloyand superimposed cooling curves,demonstrating the dependence of the finalmicrostructure on the transformations thatoccur during cooling.

Alloying element and critical cooling curve rate• Carbon and other alloying elements also shift the pearlite (as well as the proeutectoid phase) and

bainite noses to longer times, thus decreasing the critical cooling rate; one of the reasons for alloying steels is to facilitate the formation of martensite so that totally martensitic structures can develop in relatively thick cross sections;

• Fig.45 shows the continuous-cooling transformation diagram for the same alloy steel for which the isothermal transformation diagram is already presented in Fig.41, here below reported.

Fig.45 - Continuous-cooling transformation diagram foran alloy steel (type 4340) and several superimposedcooling curves, demonstrating dependence of the finalmicrostructure of this alloy on the transformations thatoccur during cooling.

Fig.41bis - Isothermal transformation diagram for an alloy steel (type 4340).

HEAT TREATMENT OF STEELS: quenching• The optimum properties of a steel that has been quenched and then tempered can

be realized only if, during the quenching heat treatment, the specimen has been converted to a high content of martensite;

• During the quenching treatment, it is impossible to cool the specimen at a uniformrate throughout—the surface will always cool more rapidly than interior regions.

• Therefore, the austenite will transform over a range of temperatures, yielding a possible variation of microstructure and properties with position within a specimen.

The successful heat treating of steels to produce a predominantly martensiticmicrostructure throughout the cross section depends mainly on three factors: (1) the composition of the alloy, (2) the type and character of the quenching medium, and (3) the size and shape of the specimen.

Hardenability of steels: influence of chemical composition on quenching cooling rate• The influence of alloy composition on the ability of a steel alloy to transform to martensite for a

particular quenching treatment is related to a parameter called hardenability.• We test this attitude by a standardized test, named the Jominy end-quench test.

Fig.47 - Schematic diagram of Jominy end-quench specimen(a) mounted during quenching and (b) after hardness testingfrom the quenched end along a ground flat.

Fig.48 – The hardenability curve

Fig.49 - Correlation of hardenability and continuous-cooling information for an iron–carbon alloy of eutectoidcomposition.

Hardness

Martensite

Bainite

Perlite

Ferrite

Hardenability curves for variour steels

Fig.50 - Hardenability curves for five different steel alloys, each containing 0.4 wt% C. Approximate alloy compositions (wt%) are as follows: 4340–1.85 Ni, 0.80 Cr, and 0.25 Mo; 4140–1.0 Cr and 0.20 Mo; 8640–0.55 Ni, 0.50 Cr, and 0.20 Mo; 5140–0.85 Cr; and 1040 is an unalloyed steel.

Industrial quenching problems: distortions and quenching cracks

Log t

Ms

T

Mf

A3A1

∆1 ∆2

Water quenched

Oil quenched

Figure 3.39 - Comparison of two possible quenching operations for a high hardenability steels. Both the water and oil quenching is possible, in terms of martensite production in part section. The faster is the quenching operation, the larger is the instantaneous differences between surface and inner section temperatures.

Increase of toughness in quenched and tempered structure

Figure 3.41 - Stress–strain diagram of a steel after different heat treatments. 1, Normalized; 2, hardened; 3, hardened and tempered.

Section sensitivity to hardness cross-section distribution of unalloyed steels

Figure 3.46 - Hardness distribution (measured) on the cross section of bars of different diameters made of unalloyed steel (0.52% C, 0.24% Si, 0.90% Mn, 0.06% Cr) quenched in conventional hardening oil from 860°C

Figure 3.47 – Hardness distribution (measured) on the cross section of bars of 10–50-mm diameters made of unalloyed steel (0.52% C, 0.24% Si, 0.90% Mn, 0.06% Cr) quenched from 860°C in water containing 5% Na2CO3.

Applicability of steel grades for hardening and tempering

Figure 3.48 – Applicability of steel grades for hardening and tempering according to required strength level and bar diameter. Steel designations according to EN 10083.

The manufacturing cycle of quenching and tempering steels

Control of quenching distortion

Fig.3.58 – Control of quench distortion by additional material left before the quenching.

Machining operations(e.g hot forging)

Quench

Temper (∼600°C)

Finishing operations(e.g. surface finisihing,

hole drilling)

Full annealing (eventuallyNormalizing + soft annealing)

Machining operations

Q+T: Basics of criteria selection (1)

Metallurgy factors:• Low carbon increases toughness and weldability, but with

low mechanical strength (e.g. C25; C25;C30);• Increasing carbon content (e.g. C50, C55, C60) determine

higher mechanical strength, but it lowers toughness;• Increasing alloying elements, especially those ones with

great affinity to carbon (e.g. Cr, Mo) allows to keep carbon content at lower values than the range 0.45-0.50% and to achieve high mechanical strength (e.g. 38Cr2; 35CrMo4; 42CrMo4).

• If high hardness is required, the carbon content shall be increased (typically at 0.50% or higher); toughness decreases.

Q+T: Basics of criteria selection (1)

• Technology factors:• Alloying elements affect positively hardenability; the

hardenability influence the critical diameter Dc; • Design factors:• When the homogenized tempered martensite structure is

targeted in a large part section, high-alloyed steels (e.g. 39NicrMo3, 30CrNiMo8, 40NiCrMo7) shall be preferred.

• Economic factor:• Percentage of alloying elements determine raw material

price of steels (Ni is one of most expensive and it is added to decrease the ductile-to-brittle transition temperature).

Self-quenching steels

The UNI EN 36NiCrMo16 is a special steel of the category quenching and tempering type series.Historically is has been referred as an air-quenching steel. Such a steel has capability to be quenched in air

The Jominy curve and properties at varying sections

YS min. UTS

Table 3.6 - Jominy test results and main mechanical properties for UNI EN 36NiCrMO16 round bars, various diameters.

Limits for chemical composition and manufacturing cycle

It is required: 5≤ (%C+%Cr+%Ni) ≤7

Machining operations(e.g hot forging)

Quench

Temper (∼200°C)

Finishing operations(e.g. surface finisihing,

hole drilling)

Full annealingPre-heat treatments

Machining operations

Heat treatments

Finishing operations

Figure 3.59 – Manufacturing cycle for an air-quenching UNI EN 36NICrMo16 part.

Machining operations

Criteria for selection

• It is high expensive steel (due to high Ni content);• Despite a raw material high cost that, sometimes a pay off is

possible when the part to be constructed is at very high added value (e.g. specific customized part)

• May be a safe-solution for a non-series product, with large sections and very complicated geometry (distortion not fully predictable);

• Request for very high strength coupled with very high toughness;

• Furthermore the presence of Ni in high percentage allow such a construction high-strength steel to be used at low temperature

Spring steels classification

A classification by Steel Grade is the following:

Carbon Steel Low Alloy Steel

Si–Mn Steel Cr–V Steel Si–Cr Steel

High-Carbon Si–Cr Si–Cr–V SteelSi–Cr–Ni–V Steel Si–Cr–V–Mo Steel

Heat treatments for steels and manufacturing cycle of springs (1)

Fig. 3.61 – Cold coiling of cold wound spring.

Fig. 3.62 – Coiling of hot wound spring.

Heat treatments for steels and manufacturing cycle of springs (2)

Cut to lenght

(Hot) Coil

Quench

Temper (∼450°C)

Preset (if required)

Shot peen

Grind (if required)

Full Annealing or shperoidization(Normalizing + soft annealing

if possible)Pre-heat treatments

Machining operations

Heat treatments

Finishing operations

Cut to lenght

(Cold) Coil

End-forming

Stress relieve

Shot peen

Grind (if required)

Cold WoundSpring

Hot WoundSpring

Patenting

Pickling

Full Annealing or shperoidization(Normalizing + soft annealing

if possible)

Fig. 3.63 – Manufacturing cycles of cold and hot wound spring.

Patenting and Pickling

Patenting: Heat treatment process eventually conducted to restore ductility resources of materials that could be highly work hardened during cold coiling steps;Pickling: Pickling, either by hydrochloric acid or sulfuric acid solution, is generally provided on patented wire and hot-rolled rod to remove their surface scale

Shot peening

Fig. 3.51 – Shot peening: a) process; b) scheme of effect on peened surface.

(a)

(b)

Fig. 3.52 – Compressive stress induced in the peened surface of a shotpeened beam (A) prevents tensile stresses from occurring after a bending moment is applied to the part.

Criteria selection

• Increasing in alloying element content is key cost-driver, • Carbon steel are used when reduced section have to be

realized and toughness is not crucial. Carbon steel with medium and high carbon content are particularly indicated for cold-worked springs.

• For hot worked large section size springs, Si and Si-Cr alloyed steel are used (increased YS and hardenability of steel).

• In case high toughness value are targeted, e.g. safe component for transportation, Ni- alloyed steels are used.

Construction Special Steels:Surface hardened steels, carburizing

steels, nitriding steels Fabrizio D’Errico

Politecnico di Milano Dept. Mech. Engineering

Approches for design requirements

To satisfy design requirements, high toughness at core and high hardness onto surface, 2 approaches can be followed:

• Water-quenching the surface of medium carbon steel to produce fresh BCT lattice martensite (non tempered martensite ) it does not affect chemical composition (i.e. surface hardening);

• Diffuse some elements on surface of a base steel, so to strengthen top layers by specific metallurgy phenomena it does affect chemical composition (i.e. thermochemical treatments)

Surface hardening steels

Surface Hardening by Localized Heat Treatment

Objective of surface hardening is to austenitize the steel at and near the surface and then rapidly quench by water to produce as-quenched martensite.

Occasionally a low temperature tempering – not higher than 100°C - is allowed to relief residual stresses.

Surface hardening techiques (1)

Flame Hardening, consists in surface heating by a gas flame created by burning acetylene, propane, or natural gas.

It realizes case depth till 5-8 mm deep, cheaper but difficult to control uniform case depth;

Surface hardening techniques (2)

Induction hardening technique, heat is supplied by surrounding the part with an inductor coil carrying a high-frequency current in the range of 2 to 500 kHz.Heat released is Q∝I2·R·t (Joule heating) and it is finely controlled by frequency; case depth ranges few millimeters to about 5 millimeters.

Fig. 3.53 – Principle of induction heating.Fig. 3.54 – Induction hardening onto a shaft. Coil and water jet system are synchronized in their relative movement with the heated shaft.

Surface hardening techniques (3)

Laser hardening, is a highly selective hardening technique in which a spatially well-defined beam of laser light is absorbed near the surface, causing rapid heating;

Carburizing steels

Design aspects of carburizing

Typical case depths range from few millimeters (e.g. around 1-2 mm) to 3-4mm.Carbon content on top layers is targeted around 0.8-1.1 wt.%, and because of the low tempering temperature, surface hardness usually obtained is about 850-900 HV (about 64-63 HRC).Effective case depth both carbon profile and hardenability of base steel.

(mm)

HV

Core hardness

550 HV

Effective depth

Figure 3.57 – Typical hardness profile for carburized part and definition of effective depth.

Criteria for carburizing steel selection

Main criteria for orienting choice of carburizing steels are design factors like top surface hardness, effective case depth and hardened case toughness, raw material and process costs.

Nitriding steels

Nitriding thermochemical process

• Nitriding pursues similar objective of carburizing; increasing hardness onto surface by locally changing chemical composition of the base steel;

• The chemical specie which is absorbed onto surface is nitrogen N;

Nitriding metallurgical phenomena

Ammonia atmosphere (gas nitriding) provide ammonia that dissociates for catalytic effect onto steel surface:

As the N is absorbed and it diffuses within the part, it combines with Fe to form small particles we call nitrides;Randomly dispersed into matrix, nitrides are capable to intercept dislocation, providing a precipitation hardening strengthening effect.

NH3 N+3H

Metallurgical aspects of nitriding (2)

DIFFUSION LAYER

POUROS ZONE

COMPACT ZONE

Figure 3.60 – Nitrided layer structure: a) simplified scheme of surface structures; b) metallography at Scanning Electron Microscope of outer layers.

• Nitrogen absorbed onto surface form compound layers, nitride ε (Fe2N) and nitride γ’(Fe4N) with minor presence of complex nitrides if alloying elements when alloying element with great affinity with nitrogen are present (mainly Ti, Al, V, Cr, Mo).

• The continuous decomposition of γ’(Fe4N) at interface with diffusion zone feed N for diffusion into solid solution Fe-α and formation of complex nitrides, when alloying element with great affinity with nitrogen are present (mainly Ti, Al, V, Cr, Mo).

ε (Fe2N)

γ’(Fe4N)

N

Fe(MxNy)

Design features of nitriding parts

Low treatment temperature (below euctectoidic temperature of Fe-N phase diagram) causes the diffusion of nitrogen is low.

Commonly, case depth is less than 0.80mm, even though nitriding times can exceed 100 h.

By prolonged nitriding time, nitriding has inherent process cost higher than carburizing, to develop same case depth;However surface hardness can achieve very high values(900HV to 1200 HV, the latter when Al is added);

Diapositiva numero 1Atomic arrangements of metalsReal configuration of a unit cellInterstitial sitesImperfections in the Atomic and Ionic Arrangements: Point defectInterfacial defects: grain boundaryMechanisms of diffusion into metalDiapositiva numero 8Diapositiva numero 9Dislocation movement: atomic scaleDislocation movement: microscopicMicrostructures of deformed grainsStrengthening mechanisms in metalsSoftening in metalsDEVELOPMENT OF MICROSTRUCTURE IN IRON–CARBON ALLOYS�The PERLITE structure in euctectoidic systems�Diapositiva numero 16Diapositiva numero 17A diagram showing the relationship between carbon content, microstructure and mechanical properties of EQUILIBRIUM STRUCTURES (Ferrite , Perlite , Cementite) in plain carbon steelsDiapositiva numero 19Use of TTT diagramThe Bainite structure in steelsNon-equilibrium structure MartensiteDiapositiva numero 23Tempering of quenched martensite CONTINUOUS-COOLING TRANSFORMATION DIAGRAMS�The euctectoidic steels�Critical cooling rate Alloying element and critical cooling curve rateHEAT TREATMENT OF STEELS: quenchingHardenability of steels: influence of chemical composition on quenching cooling rateDiapositiva numero 30Hardenability curves for variour steels Industrial quenching problems: distortions and quenching cracksIncrease of toughness in quenched and tempered structureSection sensitivity to hardness cross-section distribution of unalloyed steelsApplicability of steel grades for hardening and temperingDiapositiva numero 36Control of quenching distortion Q+T: Basics of criteria selection (1)�Q+T: Basics of criteria selection (1)Self-quenching steels�The Jominy curve and properties at varying sectionsLimits for chemical composition and manufacturing cycle�Criteria for selection �Spring steels classificationHeat treatments for steels and manufacturing cycle of springs (1)�Heat treatments for steels and manufacturing cycle of springs (2)�Patenting and Pickling�Shot peening Criteria selection�Construction Special Steels:�Surface hardened steels, carburizing steels, nitriding steels Approches for design requirementsDiapositiva numero 52Surface Hardening by Localized Heat TreatmentSurface hardening techiques (1)Surface hardening techniques (2)Surface hardening techniques (3)Diapositiva numero 57Design aspects of carburizingCriteria for carburizing steel selectionDiapositiva numero 60Nitriding thermochemical processNitriding metallurgical phenomena Metallurgical aspects of nitriding (2)Design features of nitriding parts