Stiffness of the Material and Surface Roughness: Ra Chart, Symbols and Metal Bead Blasting Guide

Two requirement families decide whether delivered stainless parts perform: the stiffness of the material and the condition of the working surface. One governs behaviour under load, the other governs friction, corrosion initiation, coating adhesion and appearance. Treating them as separate line items is the most common reason a first article fails inspection.

The stiffness of the material is resistance to elastic deformation, expressed as Young's modulus. For austenitic stainless steel it sits near 193–200 GPa; duplex and ferritic grades reach roughly 200–220 GPa. Switching from 304 to 316L therefore changes corrosion resistance far more than rigidity — grade substitution is not a stiffness strategy.

Geometry drives deflection instead: bending stiffness scales with the cube of thickness. Doubling plate thickness multiplies rigidity by about eight. In practice this means selecting from a real thickness envelope first, for example our 0.3–50mm hot rolled stainless steel plate or the 316L cold rolled plate range, then confirming the load case.

• Load case: point, distributed or pressure, with magnitude and position.
• Span and edge support: simply supported, clamped, or over stiffeners.
• Allowable deflection: for example L/250, or a fixed millimetre value at sealing faces.

Ra is the arithmetical mean deviation of the assessed profile: one number summarising average peak-and-valley height. Two surfaces with identical Ra can behave differently, so always pair the value with the process that produced it.

Treat this surface roughness Ra chart as reference only, and always state the cutoff length and instrument class in the acceptance clause.

The surface roughness symbol follows ISO 1302. The basic form is a 60° tick; adding a horizontal bar to the long leg means machining is required, a small circle in the V means material removal is prohibited, and a bridge across both legs means any process. The Ra value sits above the long leg, normally without units.

• State the standard explicitly: ISO 1302 with Ra, or ASME Y14.36.
• Use a blanket note, such as all unmachined surfaces Ra 3.2 µm max.
• Never mix RMS and Ra on the same drawing.

Metal bead blasting projects fine spherical glass or ceramic bead at controlled pressure to create a uniform, non-directional matte surface. Spherical media peen rather than cut, so there is little risk of embedding sharp fragments. It removes heat tint and weld discoloration, blends repair areas, and prepares a consistent base for coating or PVD vacuum coated colour sheet.

Write into the order: media type and size, target Ra of 1.0–3.5 µm, masking of sealing faces and bores, and passivation after blasting. For slip resistance instead of cosmetics, compare the 304 chequered anti-slip plate.

A polishing surface is built by progressive abrasive steps, each removing the scratch pattern of the previous one: 180 grit to flatten, 320 and 600 grit to refine, then compounds and felt-wheel polishing for 8K mirror. Never skip a grit, and never polish over embedded iron, which later blooms into rust spots. Start from a flat base such as cold rolled stainless sheet, and specify the mirror grade in Ra, not by eye alone.

We supply stainless sheet, plate, coil and finished parts with the stiffness of the material verified by certificate and surface condition defined by Ra and standard — delivered as metal bead blasting or a specified polishing surface up to 8K mirror. Browse the hot rolled stainless sheet range or send your drawing, load case and target Ra, and our engineers return a matched quotation with a roughness reference coupon and material test certificate.