By Spencer Erling,
Education Director, SAISC
Grade 10.9S HSFG hot dip galvanized bolt failures have raised their ugly heads again!
Failures have occurred using high strength friction grip (HSFG) bolts that have been hot dip galvanized or alternatively zinc electroplated. Such brittle type failures have been traced back to either Hydrogen Embrittlement (HE) or causes associated with the quality of the stock (raw) material and/or the heat treatment requirements needed to achieve mechanical properties, during and following the manufacture of grade 10.9 fasteners.
At a meeting of leading bolt manufacturers, the Hot Dip Galvanizers Association, an end user and the SAISC, proposals were discussed aimed at the modification and improvement of the specifications/ quality controls in order to eliminate the potential for any form of fastener embrittlement.
What then causes Hydrogen Embrittlement of fasteners when hot dip galvanizing or zinc electroplating? How is this potential problem to be eliminated during manufacturing and subsequent corrosion protection processes?
WHAT CAUSES HYDROGEN EMBRITTLEMENT?
Pre-treatment cleaning for both hot dip galvanizing and zinc electroplating, is normally achieved, by pickling fasteners in either hydrochloric acid (HCl) or sulphuric acid (H2SO4). This is commonly referred to as (acid) pickling. Acid pickling results in the release of latent Hydrogen ions (H+1) that can penetrate the grain boundaries of steel. H+1 ions, being unstable, combine to form stable Hydrogen molecules (H2) on the grain boundaries, (including microscopic surface cracking) leading to weaknesses and failure when high strength fasteners are subjected to tensile loads.
In SANS 10094 specification (the use of high-strength friction grip bolts), Annex B, a code of practice was established for processes of hot dip galvanizing grade 10.9S fasteners aimed at reducing the potential for HE. In terms of paragraph B.1.1 a) “to restrict the pickling times to less than 15 minutes” is detailed.
HOW CAN HE BE ELIMINATED?
As a result of recent experience it is proposed to totally eliminate acid pickling of class grade 10.9 fasteners. Pre-treatment cleaning is to be achieved by means of mechanical cleaning, i.e. shot or sand blasting.
Changing pre-treatment cleaning to a mechanical process, the generation of hydrogen ions is eliminated and so HE cannot then develop in the case of hot dip galvanizing.
WARNING! High strength fasteners that are zinc electroplated are susceptible to HE while being zinc coated. To remove the potential for HE in zinc electroplating fasteners a ‘heat soak process’ at 200°C is required to drive off hydrogen ions/molecules from grain boundaries.
OTHER POSSIBLE CAUSES OF EMBRITTLEMENT/FAILURE OF GRADE 10.9 FASTENERS
To achieve the correct mechanical properties required for grade 10.9S (HSFG) fasteners it is necessary for the stock material, which is used to produce the fasteners to undergo a heat treatment process before hot dip galvanizing. (Note: Grade 8.8 (830 MPa tensile strength) fastener material is not subjected to the same pre-galvanizing heat treatment and quality controls as fasteners that have a tensile strength requirement in excess of 1 000MPa)
It has been found that poor quality control, during this heat treatment process, which is required in order to achieve the correct hardness (and hence tensile strength and other mechanical properties required for grade 10.9 fasteners), has been a major source of the problems experienced with friction grip bolts and nuts. As you can imagine if a bundle of bolts (or nuts or both) are simply ‘chucked’ into a heat treatment furnace there is the distinct possibility that the bolts on the outside of the pile will be heated differently from those in the middle of the pile.
It has also been found that problems are often found to occur when the process has been a rushed late order resulting in a ‘special favour’ to get the bolts to site urgently. In such cases not enough attention has been paid to the detail requirements to meet the specifications. For example, bolts are not carefully packed in a furnace to ensure even heating of all bolts. Sometimes the work is done on a night shift, where controls are not as carefully managed as during a day shift.
It has been found that poor quality heat treatment can result in bolts that are so soft (ductile) and yielding that when attempts are made to tighten the bolts they stretch into a ‘coca-cola bottle’ profile or alternatively are so brittle that bolts and nuts sometimes fail long before ‘hand tightening’ has been achieved, let alone when torquing or stretching of the bolts commence.
Clearly neither of these problematic scenario bolts is suitable to do the job they have been designed for.
The best advice we can offer is ensure that:
1) Hot dip galvanized grade 10.9S bolts should be supplied by reputable suppliers.
2) Material certificates are required for stock material used to form the fasteners.
3) Certificates are in place of conformance for heat treatment that includes results of testing for mechanical properties and in compliance with ISO 898-1 for bolts and ISO 898-2 for nuts.
For class grade 10.9 bolts the following are the required mechanical
a. Tensile strength
Nominal = 1000 N/mm2
Minimum = 1040 N/mm2
b. Vickers hardness
Minimum = 320 (Brinell 304)
Maximum = 380 (Brinell 361)
c. Surface hardness
Shall not exceed 390 Vickers nor shall the surface hardness exceed the
core hardness by more than 30 Vickers points
d. Rockwell C hardness
Minimum = 32
Maximum = 39
e. Proof stress
Nominal = 900 N/mm2
Minimum = 940 N/mm2
f. Stress under proof loading = 830 N/mm2
g. Elongation after fracture = 10 min
h. There shall be no fractures in the bolt head In the case of class 10 nuts, for the range ofnut sizes commonly used in steel structures i.e.>M16 dia. < M39 dia., the requirements arethe nut shall be in a quenched and tempered state:
a. Stress under proof load = 1060 N/mm2
b. Vickers hardness min. 272 to max. 353
It should be required that three samples from each heat treated batch of fasteners are tested, by the manufacturer, to ensure pre-galvanizing compliance. A further three samples are to be tested, by the fastener manufacture, following the hot dip galvanizing operation.