Surface Engineering of Metals - Principles, Equipment and Technologies Part 4 pptx

2.25 shows the distribution of microhardness of electron beam hardened surface layers with different initial heat treatment, while Fig.. Besides a higher hardness than that achieved by c

Fig 2.26 Comparison of microhardness (a) and wear resistance (b) of different steels,

heat treated by traditional methods with those enhanced by the electron beam; N normalized, H - hardened; T - tempered, A - annealed; E - electron beam hardened (From Zenker, R [83, 86], and Zenker, R., et al [89, 91, 92] With permission.)

-Fig 2.25 shows the distribution of microhardness of electron beam hardened surface layers with different initial heat treatment, while Fig 2.26 shows maximum achievable microhardness values and wear resis- tance for similar layers Fig 2.27 shows a comparison of microhardness values for different materials after different versions of thermo-chemical treatment, with and without subsequent electron beam hardening The thickness of the hardened layer is within the range of several µm to as much as several mm.

The electron beam method may be used to harden low carbon and alloyed structural, bearing and tool steels, as well as white and gray cast irons [42- 95] All techniques are used, utilizing all types of beams.

Besides a higher hardness than that achieved by conventional ing, the electron beam allows the precise heating of selected spots, even of very small dimensions, while maintaining very close tolerances of hard- ened layer thickness and lower quench stresses, occurring only in small zones or even microzones of the treated material This method enables

harden-Fig 2.28 Micrographs of the surface of N135M steel, after electron beam treatment: a)

transformation hardened; b) surface remelted; c) remelted; d) intensive remelted; e)

with non-homogenous structure; f) with traces of electron beam path (From Zielecki,

W [26] With permission.)

– Intensive remelting, resulting in a clearly deteriorated

three-dimen-sional structure of the surface, primarily due to the formation of runs of remelted material.

– Very intensive remelting, resulting in a very clear deterioration of

surface structure (increase of waviness and unevenness) with clearly ible electron paths.

vis-It is also possible to obtain surface states which are intermediate tween remelting and intensive remelting.

be-The surface layer obtained by remelting has a three-zone structure (Fig 2.29) [26]:

1) Remelted and hardened from the melt zone, formed as the result of heating

to temperatures higher than the melting point, followed by dendritic lization of the remelted steel In consequence, carbides dissociate and carbon, along with other alloying elements, passes into solution This zone has a homogenous martensitic structure, containing all carbon and alloying ele- ments; the carbides are more refined and alloying elements are distributed more uniformly;

crystal-2) Subsurface zone, hardened from the solid, formed as the result of ing to temperatures above Ac3, allowing non-diffusion transformation of austenite to martensite This zone exhibits a structure which is like that formed in transformation hardening (see Section 2.5.1.2).

heat-3) Transition tempered zone, close to core which is formed in a manner

described in Section 2.5.1.2.

Remelting causes a deterioration of surface roughness relative to tial roughness, especially after intensive remelting It does, on the other hand, yield service properties which are better than those obtained after transformation hardening This is true especially of tribological properties [11−14, 61, 62, 72−99] The main reason for this is an increase of hardness or microhardness by between ten and several tens percent and a favorable dis- tribution of residual stresses The structures obtained are usually corrosion resistant For example, the fatigue strength of Nitralloy 135M may be higher

ini-by several tens percent (depending on treatment conditions may be up to 40%), tribological wear may be down by 70% and so may be loss due to corrosion (a 65 to 80% decrease of passivation current density is obtained) [26].

Fig 2.30 Hardness distribution in remelted and self-cooled layer of nodular cast iron.

(From Szymañski, H., et al [1] With permission.)

Fig 2.31 Hardness distribution in remelt hardened and nitrided GGL25* cast iron.

(From Spies, H.J., et al [108] With permission.)

Hardness and wear resistance of tool steels, which may even be three times higher [11−14], cause a 2.5 to 3 fold increase in service life of cold forming dies and an 80−90% increase in the life of turning tools [26, 95] Microhardness of eutectic and hypereutectic aluminum alloys may increase

by 30 to 500% and with alloying, even by 600% [95] This is one way of hardening piston rings [95] The hardness is increased and a martensitic or ledeburitic structure is obtained on nodular cast iron after remelting (Figs 2.30 and 2.31) The hardness of sintered carbides (based on TiC) is increased

by 12−30% [3] or even by 25−30% [1] Coarse grained tungsten obtains a fine crystalline structure [1].

The assortment of components hardened by remelting is the same as that

on which transformation hardening is carried out.

Glazing Glazing (vitrification, amorphisation) is a modification of

surface remelting1 In the case of remelting of very thin layers of some alloys

or of very thin coatings and their equally rapid cooling (usually in excess of

107 K/s), it is possible to obtain amorphous structures - metallic glazes These have the same chemical composition as that of the initial surface layer or coating, but a new set of properties, including electrical, magnetic (lower magnetic loss), mechanical (high hardness and tensile strength with the re- tention of ductility, high wear resistance), or chemical (corrosion resistance) [95] It is for this reason that glazing is sometimes wrongly identified with remelting [11−14] Electron beam glazing is applied to nickel and iron base alloys Obtainable layer thicknesses are 10−40 µm [109]; in rare cases they may exceed 100 µm [95].

Densifying (healing) Densifying, alternately, sealing of porous

mate-rial of either the surface layer or of a coating, consists of remelting the surface layer to a certain depth or of partial or total remelting of the coating, in order to make it very well sealed and to increase its density. 1) Metal glazes may also be obtained through heating of some alloys by an electron beam at lowtemperatures [95]

Electron beam surface remelting always causes sealing of a porous substrate

or coating but often may cause side effects It may also enable the removal of defects and homogenization of the prior treated material’s structure, result- ing in an increase of fatigue strength In the case of some coatings (e.g., tita- nium or sintered powders) it is possible to obtain an improvement of their structure and to increase their adhesion to the substrate It is effectively used

to seal plasma sprayed coatings.

Refinement and defect removal Refinement consists of brief

main-taining of the surface of the metal or alloy in the liquid state in order to degas in vacuum, in order to remove contaminants and non-metallic in- clusions, thereby improving physical and mechanical properties, such as density, impact strength, thermal conductivity or contact strength It is also possible at the same time to remove by remelting of mechanical and other defects, like casting flaws, scratches, cracks and blisters [95] Al- though the process is physically very similar to vacuum refinement of metals or alloys in electron beam metallurgical furnaces, in the case of superficial layers or coatings it is still in the research phase [18].

Productivity of remelting processes is estimated at approximately

250 cm2/min [3].

2.5.2.2 Alloying

Alloying, consisting of saturation of surface layers by alloying ents which are totally or partially soluble in the substrate material, is carried out with power densities greater than those employed in harden- ing and with longer heating times.

constitu-Alloying causes a deterioration of surface roughness relative to the

initial condition; after alloying the surface roughness, Rz depends to a great measure on the thickness of the alloyed layer, zalloy Usually,

Rz ∪ (0.05 to 0.1)zalloy [3] By the application of appropriate alloying

constitu-ents it is possible to obtain significant enhancement of corrosion resistance [102] and tribological properties.

Two types of alloying are distinguished, i.e., remelting and fusion

(Fig 2.32).

Remelting The first type of alloying consists of remelting of coating as

well as of the surface layer to a certain depth (Fig 2.32a) The thickness of

the alloying coating, zcoat, is approximately equal to the thickness of the

remelted layer, i.e., the mixing coefficient is km ∪ 0.51 The coating may be deposited by any means (e.g., by electrolysis or thermal spray) on the substrate, either sealed (e.g., as foil, strip or electroplating) or porous (e.g.,

in the form of paste or powder) With the remelting of both layers, their mixing occurs and the alloying material partially or totally dissolves in the substrate material After resolidification of the mixture, a different

1) Coefficient of mixing k m - ratio of cross-section of molten substrate material to total area of

cross-section of molten material; approximate formula: k m ∪ z m substr /z alloy

Fig 2.34 Wear resistance of: GGG60 cast iron; AlSi7 silicon-aluminum alloy, 1045structural steel (uncoated, SiC coating only, steel alloyed by Fe-SiC mixture),AlCu4Mg1* alloy*, alloyed by arc spraying of Ni + Al (alloy only, coating only, alloywith coating), TiAl6V4 alloy (without coating, B4C coating only, arc sprayed, andalloy cladded by B4C (From Zenker, R [106] With permission.)

Fig 2.35 Hardness profiles for different materials after electron beam treatment:

surface remelted GG20* cast iron, AlCu4Mg*1 alloyed by iron, 90MnCrV8 cold worktool steel, alloyed by Fe-SiC and TiAl6V4* alloy with B4C cladding (From Zenker, R.

[106] With permission.)

[104] Enhancement of anti-corrosion and, especially, tribological properties

is brought about by alloying of steel with nickel and chromium posited or thermally sprayed), as well as by boron carbides (B4C) and silicon carbides (SiC), plasma or arc sprayed (Figs 2.34 and 2.35) Besides the above mentioned, other substances may be used as alloying materials, e.g stainless steels, copper alloys, metal oxides, nitrides, borides and intermetallic com- pounds [18, 95].

(electrode-Fusion The second type of alloying consists of injecting of solid

par-ticles or blowing in of gas parpar-ticles of the alloying material into the melted pool of the substrate material Similarly to remelting, total or partial dis- solution of the alloying material in the substrate takes place, along with

mixing of the two materials (km ∪ 1) The alloying solid particles can be e.g., carbides and other compounds, while alloying gas can be e.g., nitogen (nitro- gen alloying), carbon monoxide or acetylene (alloying with carbon).

2.5.2.3 Cladding

Cladding (hardfacing, embedding, plating) consists of remelting of a

coat-ing, deposited on a substrate, or of a mechanically fed wire, or by injection into the electron beam spot of particles of the coating material which are insoluble in the substrate, e.g., particles of ceramic The substrate may be

subject to only small amount of remelting (km ∪ 0.1) or the coating may adhere to the substrate In concept, hardfacing is a process similar to overlaying or spray melting, with the difference that instead of a welding torch or a metallizing gun, the source of heat is an electron beam and that the cladding material does not dissolve in the substrate This method is used to produce heat, corrosion and wear-resistant coatings (e.g., in hydrau- lic components) and to repair worn machine components, like the working surface of turbine blades.

2.5.3 Evaporation techniques

Electron beam heating coupled with evaporation (vaporization) of the treated material may be utilized in the process of producing hard layers

by PVD, as well as in detonation hardening.

Electron beam material evaporation consists of bringing the material

to the volatile state in the form of vapors and of deposition of these pors by PVD methods on a substrate (see Chapter 6, Part II).

va-Detonation (explosive, impact) hardening consists of very rapid

heat-ing of the treated material by an electron beam of highest power density, causing the material to vaporize rapidly A shock wave is formed and its action on the treated material causes it to harden by impact [12] Com- plex structures are obtained, with different densities and different distri- bution of deformations, with microhardness which can be 3 to 5 times higher than in the initial material but can also be lower These microstructures may contain traces of hardening, recrystallization and other effects [109].

This type of treatment has not, up to now, been implemented on an trial scale [11−14].

indus-2.5.4 Applications of electron beam heating in surface engineering

For the past approx 15 years, electron beam heating has been used cessfully in highly industrialized countries, primarily to improve tribo- logical properties, less often to enhance corrosion resistance or strength [18] As an example, the German company Sächsische Elektronenstrahl GmbH in Chemnitz uses this method for surface enhancement of 120 different types of components, with a productivity of approximately 1 million parts annually [108].

suc-Fig 2.36 Placement of electron beam hardening within the manufacturing sequence.

(From Zenker, R., et al [90] With permission.)

Electron beam heating is used within a given technological cycle Fig 2.36 shows the location of electron beam hardening, relative to the entire component production cycle Special attention should be paid to the need

for demagnetization of parts prior to electron beam treatment melting techniques, as a rule, do not require final finishing treatment Techniques in which remelting occurs, on the other hand, do usually re- quire mechanical finishing treatment in order to give the treated surfaces appropriate smoothness.

Non-re-Electron beam treatment, both pulsed and continuous, may be applied

to parts of different surface roughness and shape and to different ments of components The roughness of electron beam treated surfaces should not exceed 40 µm The shape should be such that the treated sur- face may be held perpendicular to the electron beam Best cases are those

frag-of long and flat surfaces or ones with rotational symmetry (Fig 2.37).

Fig 2.37 Desired (a), partially desired (b) and undesired (c) shapes of parts for electron

beam heating (From Zenker, R., et al [90] With permission.)

Electron beam heating of surface situated not perpendicular to the beam is also possible, on condition that deviation does not exceed several degrees [90, 106] Examples of reaching different surfaces with the elec- tron beam are shown in Fig 2.38 Fig 2.39 shows an example of local hardening of a pin with a pulsed beam In order to facilitate the harden- ing process, manufacturers of electron beam heaters develop diagrams for various materials, correlating the desired hardening depth with the ap- propriate power density and heating time (Fig 2.40).

Typical examples of electron beam hardened components are fragments of automotive and agricultural machine parts, machine tool components (Fig 2.41) or tools, ball bearing races, including big size, piston rings, articulated joints, gears, crankshafts, camshafts, cams, flanges, rocker arms, rings, tur- bine blades, saw cutting edges, cutting edges of stamping dies, milling cut- ters turning tools, drills, etc [18].

Hardening is accomplished with electron beam heaters of several to eral tens of kilowatt power.

sev-The advantages of electron beam treatment include the possibility of treating surfaces which cannot be treated by conventional techniques [103, 105], cleanliness, elimination of deformations and dimensional changes, the possibility of precise, computerized control of the electron beam [17, 103], precise control of heating parameters, possibility of treating frag- ments of surfaces which are essentially finished and which have complex shapes, high degree of repeatability of results, ease of automation, possi- bility of achieving high treatment precision with tolerances of the order of several millimeters, high productivity, low energy consumption (efficiency reaching 80 to 90%) and, finally, the elimination of coolants.

Among disadvantages are the following: high investment cost of ment, limitation of application to selected shapes and relatively small loads, usually not exceeding the length of several meters, the necessity of using vacuum and to protect against X-ray radiation when the accelerating voltage used is high - approximately 150 kV [11−14].

equip-From the point of view of treatment quality, electron beam techniques are comparable with laser techniques.

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