The color of the oxide layer which forms on heating bright steel at
temperatures of the order of 200˚C to 400˚C. It is sometimes
used as an indication of the temperature when tempering hardened tool
steel, the colors changing with increase of temperature. Approximate
temperatures for plain carbon steels are listed below.
The above temperatures apply to normal tempering times, but the formation of
the oxide film, like the tempering operation itself, is affected to some
extent by time, and tempering for an excessively long period, even at a
temperature as low as 220˚C, would eventually produce a temper color
which would pass from straw,- through brown to purple. Higher temperatures
SALT BATH HARDENING OF HIGH SPEED STEELS
Molten salt baths offer the most versatile method
of heat treating high speed steels. We stress versatility because a
properly designed system provides the following typical advantages:
l. Fast heating and cooling.
2.Uniform hardness produced consistently.
3.Scale-free surfaces because of salt protection.
4.Ease of control by simple chemical titration.
5.Low distortion of non-uniform sections.
6.Low pollution of air and water.
7.Safe operation to meet OSHA Regulations.
8.Economical to set up and operate.
All known types of high speed steels can be heat
treated with molten salts, and a typical process cycle consists of the
following: Pre Heat, High Heat, Quench, Temper, Nitride
The heat treating salts used to provide the
working baths are all classed as neutral chlorides and are formulated to
be compatible with each step in the heat treating cycle. Contamination of
one bath from the other is eliminated by proper chemical balance of each
Proper control of the neutrality of each salt
bath is important to provide decarb-free and scale-free work. Simple
chemical analysis carried out on a periodic basis will determine when and
how much rectification is required. Neutrality here is defined as
"the absence of any chemical attack or decarb of the surface of the
high speed steels being processed". The condition of the operating
salt bath is measured by the amount of soluble oxides and insoluble
metallics contained therein. Rectification with chemical or gaseous
rectifiers control the soluble oxides within proper limits and a carbon
rod rectification procedure is used to remove the insoluble metallics.
All of the operating baths should be desludged
periodically to remove any insoluble materials or parts that have fallen
into the bath, and operating levels should be maintained through additions
of fresh salt to replace that lost through dragout. Rectifiers built into
the salt bath compositions can also be employed to maintain the processing
baths in proper neutral condition.
High Speed Salt Baths - Control, Analysis and
Molten mixtures of chloride salts used for
hardening steel are neutral in the sense that they do not react with the
surfaces of articles being treated. When properly balanced and maintained,
neutral salts will neither carburize, decarburize or pit the work pieces.
Since the alkalinity of the molten salt bath has a relationship to the
tendency of the bath to decarburize, neutrality has somewhat of a dual
meaning. The soluble oxide content of the bath, measured by pH or
titration of its aqueous solutions, must be kept within certain specified
Alkalinity in barium based neutral salts is
introduced by two major sources:
1. Processing of work contaminated with
carbonaceous residues such as cutting oils, quenching oils, rust
preventatives, or alkaline cleaner residues, or oxides on the surface of
2. Reaction of the bath with moisture at the air
interface according to the reaction:
BaC12 + H20 _____ BaO + 2 HC1
The alkalinity introduced from this second source
is dependent on the relative humidity and somewhat self limiting as an
equilibrium is reached. The alkalinity produced from the first source can
increase without limit, so care should be taken to insure that work
articles are as clean as possible when introduced in the molten salt bath.
In either case alkalinity is measured and
reported as soluble oxide content, i. e., barium oxide.
Soluble oxides in the molten bath function as
carbon scavengers. If at a high enough level, they will react with carbon
in the steel surface to produce decarburjzatjon. The extent of this
reaction will be dependent on the amount of soluble oxide in the bath, the
particular alloy, the temperature of the operation, and the time of
immersion. While a certain soluble oxide content in one particular
operation might produce critical "decarb", that same content
might have no measurable effect on an operation using a different alloy, a
shorter immersion time, or a lower temperature.
In addition to soluble oxide content, another
factor to contend with in the maintenance of salt baths is the
accumulation of insoluble materials, usually reported as "insoluble
oxides". These may be introduced into the bath from the following
1. Contaminates, as in the case of soluble oxides.
2. Metallics such as nickel, chromium, tungsten, vanadium, etc., from the
work articles (alloy steels), the pot, the electrodes.
3. Reaction with moisture and carbon dioxide at the salt air inter-face.
BaCl2 + CO2 + H20 ________ BaCO3 + ZHC1
4. Reaction of the soluble oxides with rectifiers to produce precipitates.
Insoluble materials, if allowed to build above certain specified limits,
may lead to soft spots on the surface of the articles being treated.
Although the chemical measurements of soluble and insoluble oxides serve
as a valuable guide in the maintenance of salt baths, they can be
correlated and confirmed by periodic empirical measurements, such as the
"razor blade" or shim test. This is especially important in
cases of erratic or contradictory results. In any given operation, data
from chemical analysis may be correlated to results of "razor
blade", coupon or specimen tests to establish the limits for that
Chemical Analysis of High Speed Salt Baths and
Other Barium Containing Heat Treat Salts
1.Weigh out a sample of about 5 grams to nearest
milligram and transfer to a 250 ml. beaker.
2. Add 100 ml. water and boil for 10 minutes to dissolve the soluble
portion of the salt.
3. Filter through ashless filter paper (#42 Whatman or equivalent).
4. Ignite residue in a tared crucible; cool, reweigh, and calculate
percentage of insoluble material. (Report as insoluble oxides).
5. Add a few drops of Phenolphthalein Indicator to filtrate.
6. Titrate with 0. IN Hydrochloric Acid to colorless endpoint.
7. Record mls. of 0.IN Hydrochloric Acid used and calculate percentage of
soluble oxides (report as barium oxide).
% Soluble Oxides (as BaO ) =
mls. 0.IN Acid x 0.7
Soluble Oxides Insoluble Oxides
Weight of Sample
Recommended General Limits
Calc. as BaO
(Barium Base) l.0 Max. 2.0 Max.
Hi Speed Salts 5.0 Max.
These limits are meant as a guide. In any
particular operation, the temperature, immersion time, alloy, and
requirement will determine specific chemical limits. Empirical testing
(e.g. "razor blade test") will help establish these.
Various methods of rectification are used,
usually two or more in conjunction to maintain the bath within the
recommended chemical limits, thus preserving neutrality.
1.Rectification by salt turnover such that a
daily addition of 5% of the total salt volume will maintain the bath in
proper chemical balance. In some specialized cases the amount of rectifier
can be raised to accommodate installations where dragout and replenishment
are less than 5% daily. Optionally the customer may add separate rectifier
2.Carbon rod rectification. This method is used
chiefly to remove metallics such as nickel and chromium oxides. Acting
through a reductive mechanism, the carbon rod causes these metals to be
deposited upon its surface. The type of carbon rod recommended is known as
Type AGR-AGX manufactured by National Carbon Company of New York City. We
suggest rods of 2 inch diameter, either 12 or 24 inches in length. The
rods should be suspended in the bath and removed periodically to scrape
away metallic particles deposited on the surface. A new rod should be
nearly completely submerged and withdrawn gradually as it wears at the
air-salt interface. It is necessary to guard against work coming in
contact with graphite particles introduced from the wearing of the rod.
3.Desludging. Insoluble materials result from
both the normal heat treatment process (salt decomposition) and
rectification. These must be removed periodically by desludging either
with a hoe, perforated basket, or similar device.
When using a rectified salt, the sequence of bath
maintenance might be as follows:
1.Desludge bath periodically (e. g. at end of
2.Add rectified salt.
3.Treat with carbon rod.
For an unrectified salt:
1.Add separate rectifier periodically (as
indicated by chemical analysis or established maintenance schedule).
2.Desludge after rectifier has melted into bath.
3.Add fresh salt.
4.Treat with carbon rod.
Originally it was stated that salt baths provide
fast heating and cooling. It is a fact that salts will heat the work 4 to
6 times faster than other heating methods. This results in short
processing cycles and increased production per unit of time with a minimum
amount of equipment.
The molten salt also provides a protective layer
on the work pieces during transfer from one bath to the other and prevents
scaling or oxidation of the work surface. All of the salts are water
soluble so that they may be easily washed off at the end of the cycle.
As all of the salt bath materials are neutral
chlorides, the ions contained in the rinse waters will be Na, K, Ca, Ba,
and Cl. The only toxic ion contained is barium and this is the only
ingredient that might need to be removed if the level present exceeds that
allowed by local, state or federal EPA regulations. If necessary, the
barium is simply removed by adding sodium sulfate in excess of the barium,
usually a two-fold excess is recommended, to the rinse water in a holding
tank and allowed to settle for two hours. The barium free supernatant is
then drawn off to the sewer and the non-toxic barium sulfate insoluble Is
disposed of in land fill or solid waste disposal areas.
Distortion during heating and cooling is also
reduced due to the buoyancy of the salt helping to support the work pieces
and fixtures while they are in the salt bath. This is especially helpful
where thin sections or long parts are being heat treated.
Equipment for utilizing salt baths is relatively
low cost and takes up a minimum amount of space as compared to other types
of heat treating equipment. Operations can be manual, semi or fully
automatic, and engineered to provide systems that are in full compliance
with OSHA requirements.
Versatility is the key word in the utilization of
molten salts for heat treating high speed steels. No other process can
match their ability to handle work loads ranging from 1 piece to 100 at a
time, provide cycles for short, medium or long soaking times, process
parts that are short or long, thin or thick, light or heavy, all at one
time, and still provide uniform, consistent, scale-free results at the
lowest possible costs.