Stainless Steel Hex Screw

Choosing the Right SS Hex Screw (Guide)
for stainless steel hex head screws / bolts (e.g. DIN 933 / DIN 931 / ISO 4017 / ISO 4014)

1. First decide the function of the hex screw
a) What are you fastening?

Steel-to-steel / steel-to-structure
→ Use hex head machine screws/bolts (DIN 933 full thread, DIN 931 partial thread) with nut + washer or into a tapped hole.

Into wood or plugs (no nut)
→ That’s usually a hex coach screw (DIN 571), not a regular hex screw.

So here we focus on machine type hex screws/bolts for metal joints.

2. Joint Type – Full thread vs Partial thread
a) Full thread (e.g. DIN 933 / ISO 4017)

Use when:

Clamping thin plates

General purpose bolted joints

When the shear plane does not pass through the thread or shear is low

Lengths are small (M6×20, M8×30, etc.)

b) Partial thread / shank (e.g. DIN 931 / ISO 4014)

Use when:

There is a shear plane and you want it on full shank, not threads

You need better alignment between two plates

Thicker clamped parts where a plain shank through both plates is preferred

Rule of thumb:

General fabrication → full thread (DIN 933 / ISO 4017) is usually fine.

High shear / structural joints → consider partial thread (DIN 931 / ISO 4014).

3. Environment & Material Grade
a) Choose stainless grade by environment

Normal indoor / mild outdoor / general industrial
→ A2 (SS 304), class A2-70

Standard choice: good corrosion resistance, economical

Coastal / marine / high-chloride / chemical areas
→ A4 (SS 316), class A4-70 or A4-80

Much better pitting and crevice corrosion resistance

Very aggressive conditions / high temperature / special media
→ Duplex, Super Duplex, or special alloys (2205, 254SMO, etc.) as per drawing/spec.

Cost-sensitive, non-critical, low-corrosion
→ SS 202 possible, but:

Lower corrosion resistance

Avoid near sea, chemicals, wash-down areas

b) Match grade with rest of assembly

Try to use same or compatible grade for bolts, nuts, washers.

Example: A4 hex screw + A4 nut + A4 washer for marine.

Mechanical Properties (Guide) – SS Hex Screws
A2-70 (approx. SS 304)

Material: A2 (similar to SS 304)

Strength class: 70

Minimum tensile strength: ≈ 700 MPa

Proof (yield) strength: ≈ 450 MPa

Hardness: roughly ≤ 210 HB (size-dependent)

Typical use:

General machinery

Fabrication

Building hardware

Indoor/outdoor non-marine applications

A4-70 (approx. SS 316)

Material: A4 (similar to SS 316)

Strength class: 70

Minimum tensile strength: ≈ 700 MPa

Proof strength: ≈ 450 MPa

Key advantage:

Better resistance to chlorides, seawater, many chemicals

Typical use:

Marine & coastal applications

Food & pharma wash-down areas

Chemical plants

A4-80 (higher strength, SS 316)

Material: A4 (SS 316)

Strength class: 80

Minimum tensile strength: ≈ 800 MPa

Proof strength: ≈ 600 MPa

Use when:

Higher clamp load is required

Joint is safety-critical and subjected to significant loading

Corrosion conditions demand 316

In practice, for many applications, the connected parts (plates, welds, threads in base metal) become the limiting factor before the stainless bolt strength.

Quick Field Selection Cheat Sheet

Environment

Normal → A2-70 (SS 304)

Marine/chemical → A4-70 / A4-80 (SS 316)

Joint Type

General clamping → full thread DIN 933 / ISO 4017

High shear / alignment → partial thread DIN 931 / ISO 4014

Diameter

Light → M6–M8

Medium → M10–M16

Heavy → M16+

Length

Bolt + nut → all parts + washer + 2–3 threads projection

Tapped hole → 1–1.5 × diameter thread engagement

Accessories

Always use flat washers, add spring/lock washers or locking nuts in vibration.