Chrome Core? 8 alloy is in a family of controlled chemistry, chromium-iron alloys which are candidates for use in magnetic components where corrosion resistance superior to that of pure iron, low carbon steel and silicon-iron alloys is desired without the substantial decrease in saturation induction associated with the 18% Cr ferritic stainless steels.

Applications could include electro-mechanical devices requiring some degree of corrosion resistance, either in service or for extended shelf life without the need for protective coatings.

Chrome Core 8 alloy has been considered for use in automotive components such as fuel injectors, fuel pump motor laminations and ABS solenoids.

Chrome Core 8 alloy exhibits no noticeable rusting in 95°F (35°C) - 95% relative humidity tests and have demonstrated corrosion resistance generally similar to 18% chromium ferric stainless steel in certain simulated alcohol-base fuel environments.

Chrome Core alloys were evaluated along with comparison materials in environments designed to simulate or exceed the corrosive effects of some methanol fuels. These included boiling corrosive water (proprietary low-pH solution containing chlorides) and a mixture of 50 percent ethanol and 50 percent of this corrosive water at room temperature. As seen in the Corrosion Test Results - Simulated Fuel Environment chart, there was very light or no significant attack of the Chrome Core alloys. Silicon Core Iron "B-FM", a material widely used in less corrosive environments, experienced considerably greater attack than the other alloys listed in the table.

Chrome Core alloys and comparison materials were also evaluated in CM85A corrosive fuel mixture ("Gasoline/Methanol Mixtures of Materials Testing", SAE Cooperative Research Report, September 1990). This was composed of 15% gasoline and 85% aggressive methanol, which contained 0.1% distilled water, 3 ppm chloride ion (NaCl) and 60 ppm formic acid. All specimens were exposed without deaeration in an autoclave at 176°F (80°C) for 250 hours. The following table illustrates that Chrome Core 12 alloy and Chrome Core 12-FM alloy approached the resistance of Type 430F Solenoid Quality. All Chrome Core alloys were superior to Silicon Core Iron "B-FM". Apparently, this test provided an oxidizing chloride environment and was, therefore, more severe than many anticipated service applications.

A second autoclave test using the same solution was performed with the air evacuated and without the Silicon Core Iron "B-FM" specimens to reduce both oxygen and iron contamination. The Chrome Core alloys and Type 430F Solenoid Quality displayed good resistance (corrosion rates of 0.2 mdd or less) in spite of the increased test duration of 763 hours.

Like most ferritic stainless steels, Chrome Core alloys will rust in neutral salt spray (fog) testing, although the degree and severity of rusting is substantially less than for either iron, low carbon steel or silicon-iron alloys.

For optimum corrosion resistance, surfaces must be free of scale and foreign particles. Passivation of Chrome Core 8 alloy parts is not currently recommended due to the potential for strong attack by the passivation solutions.

Data for fully annealed 0.250-0.625 in. (6.35 to 15.9 mm) diameter bars tested on a Fahy permeameter per ASTM Method A 341.

Due to the relatively low chromium content, Chrome Core 8 alloy will form austenite if heated to too high a temperature, and some hardening will occur if the austentized part is rapidly cooled. Consequently the best heat treatment for improved soft magnetic properties is to subcritically anneal.

The recommended heat treatment practice for Chrome Core 8 alloy is to anneal at a temperature of 780°C +/-14°C (1436°F +/-25°F) for 2 to 4 hours.

The cooling rate after the anneal is not critical although rapid cooling and quenching may induce stresses which impair the magnetic characteristics.

Any inert annealing atmosphere such as vacuum, inert gases or dry forming gas is satisfactory.

Attempts to decarburize the alloy using a wet hydrogen atmosphere are not recommended.

Similar heat treating practices can be used to soften the alloy for further forming.

Chrome Core 8 alloy can be blanked, formed and cold drawn. Cold work will increase the hardness.

Following are typical feeds and speeds for Chrome Core 8 alloy.

When using carbide tools, the surface speed can be increased between 2 and 3 times over the high-speed suggestions. Feeds can be increased between 50 and 100%.

Figures used for all metal removal operations covered are average. On certain work, the nature of the part may require adjustment of speeds and feeds. Each job has to be developed for best production results with optimum tool life. Speeds and/or feeds should be increased or decreased in small steps.

Chrome Core 8 alloy can be satisfactorily welded by shielded fusion and resistance welding processes. Oxyacetylene welding is not recommended. When filler metal is required, consider AWS E/ER430. Post weld heat treatment is desirable for toughness and magnetic performance. Use of austenitic stainless steel filler metal is not recommended due to the magnetic air gap created.