galfan to grit blasting

8/17/2019 Galfan to Grit Blasting

1/10

frosting.

Any surface finishing treatment which produces a surface with a fine matt appear-

ance,

e.g., blasting, brushing, barrelling and etching. Som etimes applied to glass for dec o-

rative affect.

fused spray coating.

See fusion hard facing alloys.

fusion coatings.

See fusion hard facing alloys.

fusion hard facing alloys.

Also known as 'self-fluxing overlay coatings'. Applied to an

object by firstly thermospraying and secondly fusing with an oxyace tylene torch or an RF

induction coil, which 'wets' the coat to the substrate. This produces a coating that is

metallurgically bonded to the substrate and is free of microporosity. H ence, it is im pervious

to corrosive fluids. This two-step method of application is known as the 'spray and fuse

pro ce ss'. There are various alloy types, the most important of which are based on the N i -

C r- B -S i- C system; depending upon the exact alloy compo sition, they melt in the range of

980 to 1 200

0

C. The constitution of the N i- C r- B -S i- C coatings are complex, but frequently

contain relatively large Cr

7

C

3

carbide particles (-1 0 -1 00 |nm) in a nickel rich matrix. Som e

compositions also produce coatings that contain chromium borides. The coatings show ex-

cellent resistance to abrasion wear (under light loading) and are reasonably effective in

resisting the conjoint action of corrosion and abrasion, e.g., in certain marine applications.

Another composition contains additives of coarse (~150|Hm) tungsten carbide particles which

serve to improve abrasion resistance even further.

GaIfan.

The coating produced by Galfanising.

Galfanising. A method of hot dip coating whereby steel, typically in sheet from, is im-

mersed into a bath of molten Zn-5wt%Al alloy held at approximately 450

0

C. Under opti-

mal processing conditions, a coating free from from massive interfacial intermetallics, is

produced. Instead, a fine lamella microstruc ture, con taining Tj-Zn (a solid so lution of iron in

Zn) plus 0-FeAl

3

and T]-Fe

2

Al

5

intermetallics, is formed throughout the coating. This m ethod

has been developed as an alternative to galvanising. Also see galvaluming.

galling. See

seizure.

galling resistance. See seizure resistance.

galvalume.

The coating produced by galvaluming.


2/10

galvaluming. A method of hot dip coating whereby steel, typically in sheet from, is im-

mersed into a bath of molten 5 5 A W 3 .4 Z n -l .6Si (wt-% ) alloy held at approximately 610

0

C.

The role of the silicon is to retard rapid reaction between the bath and ferrous alloy substrates,

which otherw ise would prod uce a coarse distribution of intermetallics. Also see galfanising.

galvanic cell. An electrochemical cell having two dissimilar electrical conductors as elec-

trodes.

galvanic contact plating. See contact plating.

galvanic corrosion. See

bimetallic corrosion.

galvanic current. See

bimetallic corrosion.

galvanic series. A tabulation of metals and alloys in order of their relative poten tials in any

given environment; but usually sea water. Noble metals like platinum, gold and titanium

appear at the top of the listing while base metals like zinc and magnesium appear at the

bottom.

galvanising.

See

hot dip galvanising

galvannealing. See

hot dip galvanising

gaseous alum inising or gaseous calorising.

Carried out at temperatures of 95 0-1 05 0

0

C in

a gaseous medium of aluminium trichloride and hydrogen. Aluminium is reduced at the

component surface via the reaction:

2AlCl

3

+ 3H

2

—> 2Al + 6HCl

The alum inium is diffused into the metallic components forming various intermetallic co m-

pounds. Forced circulation of the gases improves uniformity of the treatmen t throughout a

production charge. The process plant required is practically the same as that used for CV D.

Also see

aluminising.

gaseous austenitic nitrocarburising.

Conducted in sealed quench furnaces using a mix-

ture of endothermic and am monia gases and generally carried out at ~ 700

0

C. Typical trans-

formed austenite case thicknesses are in the range of 50-200|Lim with a Vickers hardness

-750-900 kg/mm

2

.

Case strength is developed by quenching (to obtain a nitrogen-carbon martensite) after

nitrocarburising which is further enhanced by ageing to decompose retained austenite to

nitrogen bainite. Apart from improving contact load resistance, fatigue strength can be im-

proved by as much as 100%. Proprietry gaseous austenitic nitrocarburising treatments are

marketed as

Nitrotec C, Alpha Plus

and

Beta.


3/10

gaseous bond ing.

'Bonding ca r r i ed ou t in a gaseous medium' - IFHT DEFINITION.

Bonding carried out in a gaseous medium containing, typically, boron trichloride (BCI3)

mixed with hydrogen and (sometimes) nitrogen. The process maybe carried out at

atmospheric or sub-atmospheric pressures. Temperatures and times are the same as those

cited under

bonding.

Also see the review: P . A .Dearnley and T. Bell,

Surface Engineering,

1985 ,1 , (3), 203-217.

gaseous boroaluminising. A multicomponent bonding technique resulting in the simulta-

neous diffusion of boron and alum inium into a steel surface.The gaseous medium is formed

by passing HCl gas through a pack containing ferroboron, aluminium and silicon carbide.

The process is conducted at temperatures -950-1100

0

C for up to 3.5 hours. It is less often

practised than the equivalent pack method and does not provide the optimal distribution of

boron and aluminium, which only

sequential

diffusion can provide. How ever, there is no

reason why this process could not be appropriately developed to enable sequential diffu-

sion. Also see multicomponent bonding.

gaseous calorising. See

gaseous aluminising.

gaseous carbonitriding.

'Carboni t r iding carr ied out in a gaseous medium' - IFHT DEFINI-

T I O N .

Essentially a modified form of gaseous carburising in which ammonia is introduced into

the carburising atmo sphere . Only sufficient amm onia is added to obtain the required

improvem ent in case hardenability. How ever, slightly higher nitrogen levels can enable an

overall improvement in temper resistance. Also see

carbonitriding

for general comments

on treatment temperature and case depth.

gaseous carburising

'Ca rbur i s ing car r ied ou t in a gaseous m edium ' - IFH T D EF INIT ION .

Probably the most popular industrial carburising method, usually carried out at tempera-

tures ~925-950°C. Batch or continuous furnace designs are the most commonly used. Com-

pared to plasma and vacuum carburising, this process lies closer to thermodynam ic equ ilib-

rium. No netheless, although possible, thermodynam ic equilibrium rarely prevails in indus-

trial gaseous carburising systems, and the use of thermodynamic theory has limited useful-

ness for process control pu rposes.

Endothermic gas (endogas) is the principal carbon source used in gas carburising. It is a

mixture of carbon monoxide( 15-20%), hydrogen (35-45%) and nitrogen (35-45%) w ith

smaller amounts of carbon dioxide(0 -l% ), methane (0.5-1.5%) and water vapour; it is gen-

erated on site by the combustion of air-propane m ixtures and has a carbon potential ranging

between 0 .35 and 0.50w t-%C . Before entering the furnace an addition of methane (CH4),


4/10

propane (C3H8), butane (C4H10), methanol (CH

3

OH) or ethanol (C2H5OH) is added which

further increases the carbon potential to a level required for carburising, usually around 0.8

wt-%. The endo gas is said to act as a carrier ga s for the hydrocarbon additive. Liquid

additives like methanol, or more complex proprietry mixtures (like glycols, ketones and

benzene) are admitted as droplets which fall onto a heated plate, inside the furnace, and are

vapourised and carried into the furnace by the endogas. The latter procedure is so metimes

called the drip feed method.

Since many gases exist in the furnace atmosphere there are several reactions that take place.

Indeed, the precise reactions are still a subject of some contention. The following, how ever,

are believed to be the main ones involved in the mass transfer of carbon to the steel surface.

(1)

(2)

(3)

(4)

Although all the above reactions have been shown moving in the direction of carbu rising, it

should be appreciated that they are all of the reversible type. Reaction 1 is som etimes

known as the Boudouard reaction, while reaction 2 is called the water gas reaction. Since

the water content of the furnace gases can be determined by using the dew point m ethod, it

is possible to estima te the carbon poten tial of the furnace. Carbon po tential is then fine

tuned by varying the quantity of hydrocarbon additive. Greater precision in the measure-

men t of carbon potential can be obtained by using an

infra-red gas analyser

w hich is able to

directly m easure the quantity of CO and CO2. In fact from equation 1 it can be appreciated

that the carbon potential is proportional to (pCO)

2

/pCO2. Yet another m ethod of asssessing

carbon potential is to use a zirconium oxygen sensor (sometimes called an oxygen probe)

which detects oxygen generated via reaction 4 . Here, the carbon potential is proportional to

(pCO)/p(O2)

1/2

. Also see

zirconia oxygen sensor.

gaseous chromising

'Chromising carr ied out in a gaseous medium'

—

I F H T D E F I N I T I O N .

In gaseous chromising a chromium halide, usually CrCl2, acts as the chromium source. It is

reduced in the gaseous state by hydrogen:

and also at the steel substrate surface (by an exchange reaction):


5/10

CrCl2 is often generated by passing HCl over chromium powder heated at 850-1000

0

C,

which is connected directly to the chrom ising vessel. Gaseous chromising is carried ou t at

850 and 1050

0

C for durations up to 12 hours. Chrom ium potential can be more precisely

controlled com pared with the pack m ethod. It is important to control the rate of CrCl2

reduction such that it does not exceed the rate of chromium solution by the substrates,

otherw ise a surface layer of uncombined chromium will be produced. The rate of chromium

solution decreases as the carbon content of a given steel substrate increases.

gaseous nitriding

'N i t r id ing ca r r i ed ou t in a gaseous medium' - IFHT DEFINITION.

In conventional gaseous nitriding, pure anhydrous ammonia acts as the nitriding medium

and a single treatment temperature, typically ~500°C, is usually em ployed. The am monia

is catalytically dissociated as it passes over the prior hardened and tempered low alloy steel

charge, according to the reaction:

In industrial furnaces, additional dissociation takes place on the chamber walls and associated

metallic parts. Nitrogen potential is controlled by controlling the flow rate of ammonia

over the charge; this controls the amount of dissociation. A level of 15 to 30% dissociation

is usually m aintained in the exhaust ga s. Now adays dissociation is best measured using an

infra-red gas analyser, although, absorbtion pipettes are still in use. At the beginning and

end of the treatments, during heating and cooling, it is usual to purge the system with nitrogen.

It has been experienced that the regulation of nitrogen potential through the flow rate of

ammonia alone is too crude to enable precise control of nitrogen poten tial, e.g., as is required

in bright nitriding. A far better option is to mix hydrogen with ammonia and to control

nitrogen p otential by controlling the ratio of these two gases. This principle was first

demonstrated in laboratory experiments by Lehrer (circa 1930) but it was not until the

1970s that the same principle was implemented on an industrial scale. How ever, a similar

effect was exploited by the Floe Process (also called double-stage nitriding), developed in

the USA , circa 1948. This proceeds like conventional gas nitriding for the first few hours,

after which dissociated ammonia is added to the anhydrous ammonia to achieve exhaust

gas dissociation levels of 75 to 80% , i.e., the second part of the nitriding c ycle is conducted

at considerably reduced nitrogen potential. A slightly higher nitriding tempe rature m ay also

be used during the second stage (~550-565°C) com pared to the first («50 0-525°C). Th is

process is still practised in the US A. Despite such refinements, however, a surprising am ount

of industrial gaseous nitriding is reliant on anhydrous ammonia alone. Also see

nitrogen

potential and zirconia oxygen sensor.


6/10

gaseous nitrocarburising

'Ni t rocarbur is ing carr ied out in a gaseous medium'

- I F H T D E F I N I T I O N .

gaseous sherardising. Sherardising carried out in a gaseous m edium containing zinc halides

at temperatures ~30O-500°C. In view of the complexity of the equipment required, gas

sherardising is not economically justified, since the products requiring sheradising are of

very low unit cost. Also see general comments under sheradising.

gaseous siliconising

'S i l iconising carr ied out in a gaseous medium'

- I F H T D E F I N I T I O N .

Carried out by the reduction of gaseous SiCU by H2 , or the pyrolysis of silane (S1H4). The

process is conducted at temperatures -8 00 -12 00

0

C for 6-7 hours. Also see siliconising.

gas plating. See chemical vapour deposition CVD).

GDOES. See

glow discharge optical emission spectroscopy.

gilding, (i) A traditional term for flash gold (24 carat) plating of fancy goods, trinkets and

giftware sold at the cheaper end of the domestic market. See gold plating, (ii) The m echan i-

cal application of gold leaf (0.076 to 0.127 \\m thick) to wooden or base metal objects.

glancing angle X -ray diffraction.

A variant on X-ray diffraction used for obtaining crystal

spacing

d)

data from the near surface. It differs from the usual X-ray methods in that the

incident X-ray beam is made to intercept the sample surface at some small glancing angle

(y). There are two metho ds: (i) the Read camera (ii) the Seeman-B ohlin diffractometer. The

former is a film method (diagram) while the latter is a specially configured X-ray goniometer

that uses a proportiona l counter to measure diffraction intensity; the output is a plot of

intensity versus 46 , where 0 is the Bragg angle. A glancing angle of 6.4° increases the X-ray

path length, in the near surface, by about nine times. Very thin (~1 |im) coatings can be

interrogated by this method.


7/10

glass structure.

See amorphous structure.

glow discharge boriding. See plasma bonding.

glow discharge carburising. See plasma carburising.

glow discharge heat treatment. See

plasma heat treatment.

glow discharge nitriding.

See plasma nitriding.

glow discharge optical emission spectroscopy GD OE S). A relatively recent (circa 1979)

optical emission spectroscopy technique that has the capability to analyse a large num ber of

elements. The design of the sample chamber is based on the Grimm lamp (diagram). The

sample is made cathodic with respect to an anular anode (~ 8mm in diameter) through

which a low p ressure flow of argon is passed. A current intensive glow discharge plasm a is

subsequently created at a pressure -1.5 torr and material is removed from the sample face

by high rate sp uttering. Whilst in-flight the excited sputtered sam ple atoms em it pho tons of

characteristic wave-length (due to electronic transitions); see diagram. The light is then

focused and passed through a collimating slit into a spectrometer w here it strikes a con cave

holographic grating, splitting the light into its component wave-lengths. The spectral detec -

tion range is -110 to 1000 nm. The system must be calibrated against primary standards,

but can routinely analyse all the major elements including, hydrogen, nitrogen, carbon,

oxygen and boron. It also has the capability to achieve rapid depth profiling, making the

techn ique of special value for the analysis of ceram ic coatings and diffusion zones. For

exam ple, it is possible to analyse 10 elements to a depth of 50 jam in less than 30 m inutes.

However, some experiences show that depth resolution is not better than 0.5|Lim.

READ CAMERA

GLANCING

ANGLE XRD

P. A. Dearnley, 1994

rays


8/10

glow discharge plasma Usually referring to a DC (direct current) glow discharge plasm a.

When a potential of a few hundred volts is applied across two electrodes held in a low

pressure (~10

2

to 10 torr) gas, partial ionisation of the gas occurs, causing ions to move

towards the cathode and electrons towards the anode. In this way electrical energy is passed

through a gas. Bernhard Berghaus w as the first scientist to systematically explore the indu s-

trial surface engineering potential of glow discharge plasmas and filed a number of sem inal

patents on sputter deposition and plasma nitriding. The essential features of a DC glow

discharge plasm a are shown in the graphics diagram. In industrial systems the chamber wall

serves as the anode which is held at ground potential. Plasma based surface engineering

processes like plasma nitriding, plasma carburising and magnetron sputtering utilise plasma

pow er densities up to 15 W /cm

2

. Such techniques require the use of pow er supplies equipped

with arc suppresion control. See arc suppression When insulating surfaces require treat-

ment radio frequency glow discharges are used. See

radio frequency glow discharge

See

also

colour section

p. A.

GDOES

TO SPECTROMETER

PHOTON

RADIATION

Ar

INSULATOR

ANNULAR

ANODE

ANNULAR

CATHODE

SAMPLE AT

CATHODE

POTENTIAL

INDOW

TO VACUUM

PUMP

P.

A. Dearnley, 1994

PHOTON

PLASMA

Ar

http://16642_cl.pdf/http://16642_cl.pdf/


9/10

glow discharge PVD See plasma a ssisted PVD physical vapour deposition).

glow discharge siliconising

See

plasma siliconising.

glow discharge titanising See plasma assisted CVD .

gold plating

Base me tals, like zinc and brass castings, are given a prior bright nickel elec-

troplate depo sit. Gold plating gilding) is costly and hence electroplating is carried out for

very short times typically ~5 to 15 seconds). Hence, the term flash gilding. For filigree

work delicate ornam ental objects made from wire) thicker coatings are needed and electro-

plating is therefore ca rried out for 20 to 30 secon ds.

gradated coating Any coating whose constitution varies continuously between successive

layers, usually enabling a progressive blending of properties between that of the substrate

and the outermost layers of the coating. Coatings of this type can be produced by thermal

spraying or plasma assisted P VD methods.

graphitic corrosion Corrosion of grey cast iron, characterised by preferential solution of

the matrix, leaving behind unreacted graphite.

V

O

L

G

E

E

T

R

I

F

E

D

P-. A. Dearnley, 1994

FIELD

VOLTAGE

ANODE

ATHODE

CROOKE

1

S

DARK

SPACE

NEGATIVE

GLOW

POSITIVE

COLUMN

GLOW DISCHARGE PLASMA


10/10

green rot.

The oxidation or carburising of certain nickel alloys at ~1000°C that results in

the formation of a green residue.

grit blasting.

A process for removing rust, paint or unw anted surface de posits (e.g., flash)

from components. A high velocity air stream is used to propel angular shaped alumina or

silicon carb ide particles onto component surfaces at high speed. The flow is directed through

a flexible hose and nozzle, enabling manipulation of the flow direction. Grit blasting is a

common preparation stage prior to thermal spray coating.

H

hard chromium plating.

See

chromium plating

hard coating. A term usually referring to transition metal carbides and nitrides deposited

by CV D or plasma assisted PV D m ethods, e.g., TiC and TiN, which have Vickers hardness

values in excesss of 2000 kg/mm

2

. The term hard coating tends to be invoked when these

coatings are being applied in order to improve wear resistance of various items, e.g., m etal

cutting tools or bio-medical prosthetic imp lants.

hardening

Any treatment designed to render an object significantly harder'

- IFHT DEFINITION.

Also see

surface hardening.

hardfacing. See

weld hardfacing.

hardness.

Qualitatively, a measure of the resistance of a surface to penetration by an indenter.

Quantitatively, a measu re of yield strength. For example, Vickers hardness Hv is related to

yield strength (G

y

) by the approxim ate relationships:

Hv

« 3 a

y

(for metals and alloys)

Hv ~

4 a

y

(for ceramics)

hardness distribution. See hardness profile.

hardness measurement.

Various methods of hardness determination e xist. These can be

broadly g rouped into: (i) static and; (ii) dynam ic hardness m ethods. In surface engineering

only static methods are used. These comprise Vickers, Knoop and Berkovich diamond in-

dentation methods. Rockwell hardness on scales A, B or C is used in accordance with the

type of material; most popular in the United States and unsuited to microha rdness determ i-

nation. See

Vickers hardness, Knoop hardness,

and

Berkovich indentation hardness.

galfan to grit blasting

Documents