A client recently complained that production cadmium-plated 12L14 steel parts, carbonitrided to Rockwell C50, were suffering from severe blistering and peeling (Fig.1). The finishing cycle (rinses omitted) consisted of soak cleaning, anodic electrocleaning, dipping in 30 percent hydrochloric acid, a second anodic electrocleaning, dipping in 10 percent hydrochloric acid, cadmium plating to a thickness of 0.0003 in., drying, and baking for 12 hr at 375 F. Blistering and peeling were evident only after the baking cycle and only on new steel stock.

In the investigative procedure to determine causes and solutions to this problem, samples of successfully plated parts and rejects were analyzed for composition by arc spectroscopy. Microscopic cross sections of the carbonitrided steel (unplated and plated) and the raw steel rod were examined at 100, 200, and 500X.

The spectrographic analysis revealed that old stock complied with ANSI 12L14 requirements, where as new stock exceeded the maximum allowable concentration of phosphorus (Table 1).

The microscopic examination showed no defects in the raw steel. The heat-treated and heat-treated/ plated samples, however, exhibited two types of defects. First, the surface of the carbonitrided steel showed evidence (Fig.2) of voids and fissures that have been described as nitrogen voids. Second, fractures varying in depth from a few 10 thousandths of an inch tp the entire thickness of the case-hardened steel were evident (Fig 3)

Substrate Makeup

By way of background, 12L14 is a resulfurized, rephosphorized steel containing up to 0.35 percent lead for machinability. It is inherently difficult to harden by conventional carburization and less so by carbonitriding.

Carbonitriding utilizes a complex atmosphere of natural gas (methane) and ammonia at high temperature (1450 F) to convert about 0.003 in. of the outer steel surface to a hard shell called a "case." The case consists of hard martensite, a complex iron-nitrogen-carbon compound, and sometimes undesirables such as retained austenite and pearlite, which are relatively soft.

The depth, structure, and composition of the case steel are dependent on numerous factors, including temperature, diffusion cycle time, gas composition, and quench type.

Although case hardening of free machining steels is easier to achieve by carbonitriding, this process does have some disadvantages. Unwelcome microconstituents can form on or immediately below the surface of the case, resulting in intergranular attack of the outer part of the case.

Surface Attack

Evidence of such surface attack was observed on the clients heat-treated steel. The result of the attack was the formation of voids and fissures, in this instance. Gases and process solutions trapped in these voids during the plating operation could be expected to push up blisters during subsequent baking.

The chief remedy in controlling surface attack is to reduce the ammonia concentration. A diffusion cycle at least half as long as the carbonitriding time has also been reported to be beneficial in minimizing surface attack during carbonitriding. (The ammonia gas is shut off and some of the nitrogen absorbed is allowed to diffuse out of the steel.)

Other factors that can contribute to surface attack are the volume of natural gas in the furnace and the diameter of the steel being hardened (the specimens submitted were of small diameteróapproximately 0.25 in.). The more natural gas in the furnace and the smaller the diameter of the part, the greater the apparent Tendency for surface attack.

This type of attack would seem to be limited to steels containing sulfur and phosphorus.2 The fact that the steel samples showing attack were too high in phosphorus content would tend to aggravate the condition described. The carbonitrided steel also evidenced some quench cracks, which likewise could be attributable to the high phosphorus content.

Whos at Fault?

The client, in this instance, was apparently not at fault. Proper activation of case-hardened steel poses difficulties for many platers. This is because the surface of the case is very stable (passive) due to the presence of stable iron-carbon-nitrogen compounds. The activation technique used by the client is typical.

However, more rigorous preparation cycles are often employed. These include: (1) etching the steel in 10 percent by vol. sulfuric acid at 50 A/ft2 at a maximum of 85 F followed by a desmutting dip in 4 oz/gal sodium cyanide plus 4 oz/gal sodium hydroxide; (2) utilizing a Woods nickel strike (30 oz/gal nickel chloride plus 10 oz/gal hydrochloric acid) first as an etch, then to apply an intermediate deposit at 100 to 200 A/ft2 prior to final plating; (3) substituting inhibited 10 percent fluoboric acid for the acid dip prior to plating, a tactic that tends to improve adhesion on leaded steels such as 12L14. These procedures typically provide adequate adhesion to sound case-hardened steel, but are not capable of preventing poor adhesion caused by a porous steel surface.

In summary, the blistering of cadmium-plated case-hardened steel was attributed to voids and fissures in the substrate caused by surface attack during the carbonitriding heat treatment and by over brittleness of the steel due to improper composition (high phosphorus).

References

1. G. Koves, Trans. ASM, 56, p. 315 (1963).

2. A.W. Douglas, W.E. Heitman, and E.S. Madrzyk, Inland Steel Internal Report, F239 Q26s, "The Effect of Tellurium on Metallurgical and Mechanical Properties of Steel."