October_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 5

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FEATURE

KEYS TO LONG-LASTING HARDENING INDUCTORS:

EXPERIENCE, MATERIALS, AND PRECISION

Valery Rudnev,* FASM, Aaron Goodwin, Steven Fillip,* WilliamWest, Jim Schwab, and Steve St. Pierre,

Inductoheat Inc.

*Member of ASM International and ASM Heat Treating Society

nduction coils are considered theweakest link in an induc-

tion hardening system, so advanced designs and precise

fabrication are paramount to ensure long life while pro-

ducing high quality treated parts.

The terms hardening inductor, inductor, induction coil,

and coil are all used interchangeably to describe the electri-

cal component that provides the induction heating effect

in an induction heating system. A hardening inductor is of-

ten simply called a coil, but its geometry does not always

resemble the classic circular coil shape. Figure 1 shows a

sample of numerous coil designs. A particular coil configu-

ration depends on several factors such as workpiece geom-

etry, temperature uniformity and required heat pattern, and

production rate, among others. Alternating current flowing

in the inductor generates a time-varying magnetic field that

provides an electromagnetic link between the inductor and

workpiece, resulting in contactless heating of either the en-

tire workpiece, or selected areas.

Coils are considered the weakest link in an induc-

tion hardening system because they carry significant elec-

trical power and operate in harsh environments exposed to

high temperatures, water, and other coolants, while being

subjected to mechanical movement and accidental part

contact. Advanced coil designs and precise fabrication can

ensure long life while producing high quality treated parts.

MATERIAL SELECTION

Copper andcopper alloysarealmost exclusivelyused to

fabricate induction coils due to their reasonable cost, avail-

ability, and a unique combination of electrical, thermal, and

mechanical properties. Proper selection of copper grade and

purity for a coil is crucial to minimize the deleterious effects

of factors that contribute to premature coil failure including

stress-corrosion and stress-fatigue cracking, galvanic corro-

sion, copper erosion, pitting, water leaks, overheating, and

work hardening. Cooling water pH also affects copper sus-

ceptibility to cracking.

Oxygen-free high-conductivity (OFHC) copper should

be specified for most hardening inductors despite its high-

er cost. Besides superior electrical and thermal properties,

OFHC copper dramatically reduces the risk of hydrogen em-

brittlement. The higher ductility of OFHC copper is also im-

portant, because coil turns are subjected to flexing and high

electromagnetic forces. The higher cost of OFHC copper usu-

ally is offset by improved hardening inductor life.

FABRICATION TECHNIQUES

Two traditional techniques used to fabricate hardening

inductors are banding and brazing of square, rectangular,

and round copper tubing. The ability to precisely and repeat-

ably fabricate banded or brazed inductors of complex geom-

etry has always been a legitimate concern, which requires an

extensive and costly validation process after installing a new

set of inductors.

Silver-base braze material is used to fill joint gaps in

brazed copper tubing. The fact that electrical and thermal

properties of pure silver are superior to those of copper has

led some coil builders and practitioners to assume that the

filler metal provides electrical contact between brazed com-

ponents as good as with solid copper, which is not the case.

Porosity and the presence of oxides and other elements

increase the electrical resistance of the brazed joint area

compared with that of solid copper. As a result, excessive

heat is generated in the copper joint area, unless the joint is

located in a portion of the coil that does not carry electrical

current. Excessive heat generation causes deterioration of

brazed joints, shortening coil life.

A complex geometry inductor that contains numer-

ous brazed joints, and 90° joints in particular, could expe-

rience impeded water flow in cooling coil turns, a problem

more likely to occur in a coil fabricated with small-diam-

eter tubing. This situation could require the use of boost-

er pumps to provide sufficient water flow to cool the coil.

However, this can be counterproductive as excessive water

pressure adds to the electromagnetic forces and thermal

stresses experienced by the copper coil, which could fur-

ther weaken brazed joints, leading to cracking and water

leaks. Also, brazed joints and the copper itself can weak-

en due to work hardening during coil service, becoming

brittle and developing fatigue cracks. Eliminating or sig-

nificantly reducing the number of brazed joints, particu-

larly in current-carrying areas, is a key factor in fabricating

long-lasting inductors.

CNC MACHINING ANDQUALITY ASSURANCE

At Inductoheat, most high power-density hardening

inductors are CNCmachined from a solid copper bar regard-

less of complexity. This repeatable machining process pro-

duces rigid, durable inductors. CAD/CAM/CNC software pro-

grams are created that provide appropriate cutter-to-copper