MATERIAL APPLICATION CONSIDERATIONS

The balance between material from which a part is made and the process by which it is manufactured will determine its performance characteristics. Some materials are more corrosion resistant than others depending on the environment in which they are used. The process by which a part is manufactured may determine strength and load bearing characteristics, which may be critical if safety is a primary requirement.

Various materials behave differently when subjected to tensile, shear and shock loads, and when used in contact with or in proximity of other materials. Use of dissimilar metals on the same assembly or vessel can result in galvanic corrosion, causing severe failure to parts within a very short time.

Many different alloys and grades of metals are used in marine applications to take maximum advantage of properties. Primary considerations which determine alloy selection include corrosion resistance and strength. Stainless Steel parts utilize two grades, 316 for casting and 304 for machining and forming.

Example 316 Stainless Steel  Ring Pull

Example 304 Stainless Steel Pad Eye 

 

BREAKING STRENGTH

Usually this type of rating will not apply to a cast part, regardless of the material, since there is no guarantee of consistency in base material throughout the part. There may be air voids and other imperfections related to the process of casting which may affect structural integrity and cannot be seen "under the skin" without cutting the part open.

Parts which may be load rated by our suppliers are forged, drop forged, stamped and machined parts. Be sure to refer to the "Breaking Strengths" section in the front of our catalog, which describes definitions of Breaking Strength and Working Load Limit (WLL).

Because a part is not load rated does not mean that the part is not usable in most reasonable applications. It is up to the end user to determine if the part may cause personal injury to a user, by their own testing and common sense evaluation of the part.

Cast parts will not typically have a Breaking Strength Rating, regardless of the material, due to the possibility of air pockets being trapped and not visible under the skin of the part. Types of casting include investment, sand and die casting. Investment casting by the "lost wax" method is used to make stainless steel parts. Sand casting is used to make gray iron, aluminum and manganese bronze items. Die casting is used to make manganese bronze and zinc based alloy fittings.

Stamped parts offer a reliable low cost alternative to cast stainless steel and brass parts. They are processed with tooling that simulates a "cookie cutter" on sheet or coil ck, and then usually formed to shape with bending dies.

Extruded brass parts are made by forcing heated material through a die which has the shape of the desired cross section. Many of these parts are then machined with additional details such as holes and slots, etc. 

Machined parts made from stainless steel and brass are cut from a solid piece of material on mills and lathes. They offer excellent durability in non-critical applications. 

Drop Forged steel and bronze parts generally offer the greatest tensile strength and best resistance to shocks and distortion. They are hot-formed in accurately cut dies, resulting in parts which are dimensionally very consistent. Some parts intended for lighter duty are made by forming and bending under pressure but without heat.

Injection Molded plastic pellets are drawn into the screw (auger) and heated to the proper melting point by heat bands surrounding the screw. The melted plastic is then injected into precision molds to form the parts. After molding, trimming off of the runner is done before assembly or sale.

Stainless Steel is passivated to remove processing impurities from the surface which may corrode. If additional processing such drilling or grinding is done to a part after it is passivated, small chips of drilling or grinding tools may be imbedded in the surface of the steel, which may themselves corrode, falsely creating the appearance that the base metal is corroding.

Grinding and Polishing is the most common means of surface finishing stainless steel and bronze items. Parts finished with this process are done by hand so there may be slight variation between parts.

Chromium Plating is a very hard and durable non-tarnishing finish which is applied to brass, bronze and zinc items. It typically includes a three step process of applying copper, nickel and chromium.

Painted finishes for marine applications are usually a two part catalyzed finish for excellent durability and corrosion resistance, usually on steel and aluminum parts.

Powder Coating is a finish which is applied by electrically charging steel and aluminum parts and the urethane powder to allow a uniform coating and then baking for a tough, durable skin.

Hot Dipped Galvanized finishes are applied by immersing parts in molten zinc and are common on cast iron parts. Zinc is a sacrificial coating which migrates to scars and scratches resulting from use to protect the base metal from contact with water and moisture.

Electrogalvanizing is a process used to protect smaller steel parts with a zinc coating.