Most parts that are hard chrome plated require some sort of surface finishing, either before plating (BP) and/or after plating (AP).
Chrome deposits with a phenomenon known as low micro-throwing power and the chrome deposit does not provide any leveling of the substrates surface. This means that the chrome deposit will not fill scratches and it will not bridge over any small voids. On thinner deposits the chrome will reproduce the finish almost exactly. Thicker deposits seem to have their surface imperfections magnified and the deposit will have a coarser RMS than the substrate had before plating. The thicker the deposit, the more noticeable this imperfection becomes. Most parts are therefore subjected to some type of surface finishing both BP and AP. The following tips may be helpful.
The vast majority of parts require polishing BP in order to bring the surface finish to the desired point. Polishing also removes surface defects and provides a better final plated finish. This polishing can be done with a wheel using polishing compounds or by belt polishing. In most cases several grit sizes are used until the final RMS is obtained. In general, the finish provided BP should be at least as good, if not better, than what is desired AP. It is also common to provide some sort of polishing AP to remove minor nodules and provide the final finish desired. AP polishing is needed more with thicker deposits and when the anode - fixture conditions are not optimized.
Glass Bead Blasting
Glass bead blasting is the preferred method of removing large surface imperfections BP. If used, glass bead blasting is done prior to polishing. Other abrasives, such as aluminum oxide, are generally avoided as these can embed particles into the surface which are frequently a cause of pitting and roughness. In some cases, liquid honing, also known as vapor blasting, is used in which super fine slurry of aluminum oxide abrasive is mixed with water and blasted against the surface under high pressure. In this instance the use of aluminum oxide is satisfactory. A glass beaded surface has a satin appearance, and will produce a satin chrome deposit unless further polished.
Dura Prep Scrubbing
BP hand scrubbing with Dura Prep, prior to reverse etching, is an excellent way to produce high quality deposits that are pit free. The dura Prep is applied as a water slurry with a pad and a scouring action. This removes the last traces of unseen metallic slivers, debris and stop-off residue. The Dura Prep is non hazardous and can be rinsed off without disposal considerations. This type of final cleaning is usually reserved for low production volumes.
Grinding is commonly performed both BP and AP. BP grinding is different from other typical grinding operations in that certain steps must be taken to ensure that the part is in the proper condition for high quality plating. The method in which the surface is brought to size is important. If a large amount of stock is to be removed, heavy cuts may be taken until the part is within 0.005" of final size. At that point, the feed rate should be reduced to only a 0.0005" removal rate per pass. This will prevent glazing over which leaves small slivers and debris on the surface. This debris, if present, will “stand-up” in reverse etching thereby producing nodules and pits in the deposit with later AP grinding.
AP grinding of hard chrome must also be done with certain precautions. A special soft wheel specifically designed specifically for chrome should be used along with plenty of coolant. Chrome grinding produces a white spark and it’s hardness causes a high wear on the wheel which must be dressed frequently. The removal rate should be limited to 0.0002 - 0.0004" per pass. Also avoid chipping the deposit on edges. The reason for the slow grinding requirement is due to the occluded hydrogen present in the deposit. If ground too fast, the excessive heat generated will cause expansion and the deposit to crack. In severe cases this can also cause a small area of the deposit to blow out. While there is a tendency to blame this on poor adhesion, the real cause is improper chrome grinding.
Baking and Shot Peening
Federal Specification QQ-C-320 outlines several preparation steps needed for plating on hard or high tensile strength steels. In brief, all steels with a hardness of Rockwell C-40 and higher should be baked, (embrittlement relieved) after plating. This is done at 375 deg. F (+/-25 deg.) for at least 3 hours, and must be done within 4 hours of plating. Springs and other parts subject to flexure must not be flexed prior to baking.
Thermal Stress Relief (also baking) is done before plating on any steel part having an ultimate tensile strength of 150,000 psi or higher. The same temperature and baking time is used. This provides relief of damaging residual tensile stresses. Steels with an ultimate tensile strength of 240,000 psi, or greater, should not be plated. When shot peening is also required, the thermal stress relief is performed first.
Steel parts designed for unlimited life under dynamic loads should be shot peened prior to plating. This peening must be done on all areas to be plated as well as on all immediately adjacent surfaces which contain notches, fillets or other abrupt changes of section size where stresses could be concentrated. Also, any steel having a hardness of Rockwell C-40 or higher should be shot peened prior to plating. Either steel or glass beads may be used. All shot, however, must be free of sharp edges. In most cases, the part is first coated with “Dyescan” and then observed under a black light, after peening, to ensure complete coverage. No operation is allowed after peening and prior to plating that may induce additional stresses into the substrate.
The surface finish may be improved after peening by polishing, honing or lapping operations. Shot peened parts should be protected with a corrosion inhibitor prior to plating if a delay is expected. All peening must be in accordance with MIL-S-13165 and MIL-R-81841.