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Stainless steel is a great material to work with. As with all materials you need to know how to treat it correctly for maximum cost effectiveness. This article summarises the typical pitfalls and remedies.
This article has a listing of terms often specifically associated with stainless steels, their processing and use. Terms listed include, active, annealing, austenite, austenitising, bright annealing, cathodic protection, chlorides, (halides), cold and hot working, corrosion, creep, deep drawing, duplex, fatigue, (endurance), ferrite, martensite, normalising, passive, passivation, permeability, pH, pickling, pinch pass, pitting, precipitation hardening, scaling temperature, sensitisation, stabilisation, stress relieving, stretch forming and tempering
A colour chart is shown for guidance on exposure temperatures on 1.4301, (304), type. The factors affecting the degree or depth of colours formed are outlined. These include steel composition, atmosphere, time and surface finish.
Heat treatment, (tempering), of austenitic strip and wire grades for enhanced mechanical properties is referenced in BS EN 10151, (strip), and BS EN 10270-3, (wire).
Stainless steels are often regarded as ‘difficult to machine’ and classed a single group of steels, based on experience with the most common austenitic types. Machining these steels using feeds, speeds and depth of cut parameters for more conventional steels can result in excessive tool wear or breakage. Although cold drawn bar can have better surface finish and accuracy of tolerances than ‘black’ bar, the relative ease of machining of the fully annealed ‘black’ bar may make this a better overall choice for the machinist or engineer. The machinability of the other stainless steel ‘families’, i.e. ferritic, duplex, martensitic and precipitation hardening is however different. A machinability index for grades such as 303, (1.4305), 304, (1.4301), 430, (1.4016), 2205, (1.4462), 416, (1.4029, 1.4005), 410, (1.4006), 440C, (1.4125) within these families is shown.
Stainless steels can be readily cut using laser methods. The properties of the steel, including magnetic permeability, should not be affected by laser cutting. Any distortion experienced during or after cutting should only be due to the relief of ‘self restraint’ on cutting and not due to any changes in the material from the cutting method.
Austenitic stainless steels are usually described as non-magnetic, with a relative magnetic permeability of around 1.0. Permeabilities above 1.0 are associated with the amount of ferrite or martensite phases present in the austenitic steel. These depend on the precise chemical composition and the effects of cold working and heat treatment.
The special compositions, non-metallic inclusion, ferrite and grain size requirements of these steel grades, D and E specified for surgical implants are outlined. Commercially produced 316 type steels cannot be used for surgical implants as the available composition and micro-cleanness information is very unlikely to meet the requirements of BS 7252-1, ISO 5832-1.
Drill and reamer tool angles are shown along with tables of suggested feeds and speeds for drilling and reaming a range of stainless steels, using either high speed steel, (HSS), or carbide tipped tools. The particular problems drilling small diameter holes in stainless steels are highlighted. The data is taken from the BSSA Stainless Steel Specialist Course, training note No.9 ‘Machining Stainless Steels’
Tables of suggested feeds, speeds and depth of cut for single point, form tool and cut-off, (parting-off), turning are shown. Tool profiles are shown and discussed along with suggested relief angles for these types of lathe turning operations. The data is taken from the BSSA Stainless Steel Specialist Course, training note No.9 ‘Machining Stainless Steels’
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