Stainless Steel Fasteners for Timber Construction

Choice of fastener material

Austenitic stainless steels are considered to have Group 4 (excellent resistance)in the ranking of the resistance of common metals to corrosion by acetic acid emitted by wood.

Where CCA preservative treatments have been applied, the timber should be aged for at least seven days before fasteners are inserted. Standard for external timber framed walls recommends austenitic stainless steel as a suitable material for:

• staples, screws, and nails for fixing sheathing and breather membranes
• nails, screws for fixing CCA treated wood
• wall ties and fixings.

Austenitic stainless steel is also a recommended material for external structures of woods such as cedar and permanent structures (painted or oiled).

Timber in immersed structures

Austenitic stainless steel fasteners are suitable for use in immersed timber structures. The choice of stainless steel grade will depend on water conditions such as:

• chloride level
• water flow rates
• temperature
• oxygen levels
• crevice conditions in the fastener system geometry.

Stainless steel grade 304 (1.4301) may be suitable for fresh water, but in higher chloride levels 316 (1.4401) is preferable. BUMAX 316L HiMo (1.4436) is considered to be the strongest of 316L classifications and able to withstand corrosion longer.  http://www.BUMAX.se/en  The conditions in tidal river estuaries and seawater where higher chlorides, low flow rates and low oxygen levels can predominate may require higher molybdenum grade stainless steels such as a 6% molybdenum grade (1.4547). Care must also be taken to avoid bimetallic (galvanic) corrosion if combinations of dissimilar metals are to be used.

Timber in swimming pool buildings

Particularly aggressive internal environments can also exist in swimming pool buildings, especially for roof fixings where applied loading can also give rise to stress corrosion cracking (SCC) failure hazards. For these applications SCC resistant grades such as:

1.4436 BUMAX www.BUMAX.se/en/

1.4547

1.4529

1.4565

should be considered.

Selecting the best material for your fastner application

Materials

Stainless Steel
Stainless steel is an alloy of low carbon steel and chromium for enhanced corrosion characteristics. Stainless steel is highly corrosion resistant for the price and because the anti-corrosive properties are inherent to the metal, it will not lose this resistance if scratched during installation or use.

It is a common misconception that stainless steel is stronger than regular steel. In fact, due to the low carbon content, stainless steel cannot be hardened. Therefore when compared with regular steel it is slightly stronger than an un-hardened (grade 2) steel fastener but significantly weaker than hardened steel fasteners.

Stainless steel is also much less magnetic than regular steel fasteners though some grades will be slightly magnetic.

18-8 Stainless
18-8 refers to any stainless steel containing approximately 18% chromium and 8% nickel. This is the most common stainless designation for hardware. For information on 18-8 stainless steel material properties see our Material Grade Identification and Properties Chart.

Steel
Steel is the most common fastener material. Steel fasteners are available plain as well as with various surface treatments such as zinc plating, galvanization, and chrome plating.

Steel fasteners are commonly available in 4 grades. Many other grades exist but are used far less often. The most common grades are Grade 2, Grade 5, Grade 8, and Alloy Steel. Grade 2, 5, and 8 are usually plated with a silver or yellow zinc coating or galvanized to resist corrosion.

Determining Bolt Grade
Bolts of different grades are marked on the head to show what grade bolt they are.

Grade 2
Grade 2 is a standard hardware grade steel. This is the most common grade of steel fastener and is the least expensive. Grade 2 bolts have no head marking (sometimes a manufacturer mark is present).

Grade 5
Grade 5 bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines on the head.

Grade 8 
Grade 8 bolts have been hardened more than grade 5 bolts. Thus they are stronger and are used in demanding applications such as automotive suspensions. Grade 8 bolts have 6 evenly spaced radial lines on the head.

Bumax88

Special 316L stainless steel bolt equivlent to a hardered carbon steel bold SAE J429  Grade 5 bolt while retaining or exceeding all characteristics of 316L stainless steel

Bumax109

Special 316L stainless steel bolt equivlent to a hardered carbon steel bold SAE J429  Grade 8 bolt while retaining or exceeding all characteristics of 316L stainless steel

Alloy Steel
Alloy steel bolts are made from a high strength steel alloy and are further heat treated. Alloy steel bolts are typically not plated resulting in a dull black finish. Alloy steel bolts are extremely strong but very brittle.

www.bumax.se or visit global spec http://www.globalspec.com/Supplier/Profile?vid=368881

Silicon Bronze
Silicon bronze, often referred to simply as bronze, is an alloy made mostly of copper and tin with a small amount of silicon. Bronze is used primarily in marine environments. It is preferred over stainless in wooden boat construction and re-fastening due to its superior corrosion resistance, and over brass due to its higher strength. Bronze is similar to copper in color and is also sometimes seen in fine woodworking where it is used for its appearance. The main drawback of bronze is its high cost.

Brass
Brass is an alloy of primarily copper and zinc. Brass is highly corrosion resistant and electrically conductive. However, its use as a fastener is somewhat limited due to its relative softness. It is used primarily for its appearance.

Aluminum
Aluminum is a light, soft, corrosion resistant metal. Like stainless steel, aluminum's corrosion resistance is inherent to the material. Therefore scratches and nicks will not effect the corrosion resistance.

Fasteners are made from a variety of aluminum alloys with elements such as manganese, silicon, iron, magnesium, zinc, copper, and silicon being added to increase strength and melting point.

Rivets are often made from aluminum alloys in the 5000 series which uses magnesium as the primary alloying element.

Coatings

Zinc Plating
Many steel fasteners are electro-plated with zinc for better corrosion resistance. Fasteners that have been zinc plated have a shiny silver or golden appearance referred to as clear or yellow zinc respectively. They are fairly corrosion resistant but will rust if the coating is destroyed or if exposed to a marine environment.

Hot Dip Galvanizing
Galvanizing is another coating involving the application of a layer of zinc. Hot dipped galvanizing puts the thickest possible coating on the metal resulting in superior corrosion resistance. Due to the thickness of the coating hot dipped galvanized bolts are not compatible with other nuts. Galvanized nuts are tapped slightly larger than other nuts to accommodate this coating. 
Hot dipped galvanized fasteners are frequently seen in coastal environments.

Chrome
Chrome is used in plating fasteners for its appearance. It provides similar corrosion resistance to zinc plating. The main drawback of chrome is the extremely high cost. If more corrosion resistance is required stainless steel may be chrome plated, preventing any corrosion should the chrome be penetrated.

Coating of carbon steel ISO: 4042 Standard Platings/ Surface Coatings

STANDARD PLATINGS: SURFACE COATINGS
ISO : 4042 (EN ISO 4042) Electroplated coatings
DIN : 267 Part 9

Electroplated coatings/Rivestimenti elettrolitici/Revêtement électrolytiques/ Revestimientos electrolìticos/Elektrolytische Überzüge Code system/Sistema di codifica/Systême de codification/sistema de codificaciòn/Kodifizierungssystem 1. Scope and field of application These technical conditions are in particular related to threaded fasteners (mainly bolts and nuts), but are also applicable to the whole range of mechanical fasteners. 2. Electroplated coatings An electrolytically applied coating shall be defined as a protective metallic layer being deposited onto the surface of metal articles by immersing these parts in an aqueous solution through which an electrical current is passed. Note: The use of the nomenclature “galvanizing” for this treatment is not correct. This information on electroplated coatings corresponds with DIN 267 Part 9 and ISO 4042. 3. Code system The electroplated coatings of mechanical fasteners are designated by a code consisting of a combination of two capital letters and a number. This callout system is built up as follows: – a capital letter for the coating metal (Table 1) – a number for the minimum layer thickness (coating structure) (Table 2) – a capital letter for the degree of gloss and after-treatment (Table 3)

	Table 1. Coating metal				Table 2. Minimum layer thickness (coating structure)
	Code					Layer thickness (coating structure) in µm
	letter	Coating metal	Symbol		Code number	1 coating metal	2 coating metals
	A	Zinc	Zn		0 1)	–	–
	B	Cadmium	Cd		1	3	–
	C	Copper	Cu		2	5	2+ 3
	D	Brass	CuZn		3	8	3+ 5
	E	Nickel	Ni		4	12	4+ 8
	F	Nickel-chrome 1)	NiCr		5	15	5+10
	G	Copper-nickel	CuNi		6	20	8+12
	H	Copper-nickel-chrome1)	CuNiCr		7 2)	25	10+15
	J	Tin	Sn		8 2)	32	12+18
	K	Copper-tin	CuSn		9 2)	40	16+24 3)
	L	Silver	Ag		1) Code number 0 applies to screw threads below
	N	Copper-silver	CuAg		M 1.6, where no specific layer thickness can be specified.
	P	Zinc-Nickel 3)	ZnNi		2) Does not apply to threaded components.
	Q	Zinc-Cobalt 3)	ZnCo		3) Not in ISO 4042
	R	Zinc-iron 3)	ZnFe
	1) Thickness of chrome layer 0,3 µm
	3) Not in ISO 4042

	Table 3. Degree of gloss and after-treatment
	Codeletter	Degree of gloss	Chromating in accordance with DIN 50 941 Process group	Self-color of chromate layer passivation by chromate
	A	mt (dull) (mat)	none 1)	none
	B	mt (dull) (mat)	B	bluish to bluish iridescent 2)
	C	mt (dull) (mat)	C	yellowish glistening to yellowish-brown,iridescent
	D	mt (dull) (mat)	D	olive green to olive brown
	E	bk (bright)	none 1)	none
	F	bk (bright)	B	bluish to bluish iridescent 2)
	G	bk (bright)	C	yellowish glistening to yellowish-brown,iridescent
	H	bk (bright)	D	olive green to olive brown
	J	gl (glossy)	none 1)	none
	K	gl (glossy)	B	bluish to bluish iridescent 2)
	L	gl (glossy)	C	yellowish glistening to yellowish-brown,iridescent
	M	gl (glossy)	D	olive green to olive brown
	N	hgl (high gloss)	none	–
	P	bel (optional)	B, C or D 3) at manufacturer's discretion	as for process group B, C or D
	R	mt (dull) (mat)	F	brownish black to black
	S	bk (bright)	F	brownish black to black
	T	gl (glossy)	F	brownish black to black
	U	all finishes		no chromate treatment
	1) In the case of Zn and Cd		3) Process groups B, C or D in accordance with DIN 50 941 only apply to
	however, process group A		cadmium and zinc coatings. In the case of other electroplated coatings, "P"
	2) Only applies to Zn coatings		in the code symbol signifies "degree of gloss optional".

	Ordering code for electroplated coatings for commercial fasteners
								Copper
		Coating		Zinc-chromated			Nickel	nickel
		Degree of gloss	Glossy	Glossy	Glossy	Glossy	Glossy	Glossy
	Nominal size	Nominal size	Color	Color	Color	Color
	metric	inch	none	bluish	yellowish	black	–	–
	< 5 mm	< 3/16 "	A1J	A1K	A1L	A1T	E1J	G2J
	5 < 10 mm	3/16" < 3/8 "	A2J	A2K	A2L	A2T	E2J	G2J
	> 10 mm	> 3/8 "	A3J	A3K	A3L	A3T	E3J	G3J

Example of coding: A3L means zinc-plating (A in Table 1) with a minimum layer thickness of 8 µm (3 in Table 2) and yellow-chromated with a glossy degree of gloss (L in Table 3). Callout Example: Hexagon bolt DIN 931 - M12 x 50 - 8.8 - A3L. Chromate (passivate) effected immediately after electroplating by short immersing in chromic acid solutions. The chromating process increases the corrosion protection and prevents a starting and a discolouring of the zinc layer. The protective effect of the chromate layer is different depending upon group of procedures (see table).

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