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 are required
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 salt composition.
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 Rectification
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 limits.
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 the work.
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 sources:
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 particular operation.
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
Weight of Sample
Recommended General Limits
Soluble Oxides Insoluble Oxides
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 as required.
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 shift).
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.