NOTE 1 - The following two paragraphs are essentially the text of the introduction of
ISO 9588 --3): (see(2)).
When atomic hydrogen enters steels and certain other metals, for example aluminiurn and
titanium alloys, it can cause loss of ductility or load carrying ability, cracking (usually
as submicroscopic cracks) or catastrophic brittle failures at applied stresses well below
the yield strength or even the normal design strength for the alloys. This phenomenon often
occurs in alloys that show no significant loss in ductility when measured by conventional
tensile tests, and is frequently referred to as hydrogen induced delayed brittle failure,
hydrogen stress cracking or hydrogen embrittlement. The hydrogen can be introduced during
heat treatment, gas carburizing, cleaning, pickling, phosphating, electroplating,
autocatalytic processes and in the service environment as a result of cathodic protection
reactions or corrosion reactions Hydrogen can also be introduced during fabrication, for
example during roll forming, machining and drilling due to the break-down of unsuitable
lubricants as well as during welding or brazing operations. Parts that have been machined
ground, cold-formed or cold-straightened subsequent to hardening heat treatment are
especially susceptible to hydrogen embrittlement damage.
The results of research work indicate that the susceptibility of any material to hydrogen
embrittlement in a given test is directly related to its hydrogen entrapment population
(type and effectiveness of traps) Therefore the time-temperature relationship of the baking
process is dependent on composition and structure of steels as well as plating metals and
plating procedures Additionally, for most high strength steels, the effectiveness of the
baking process falls off rapidly with reduction of time and temperature.
NOTE 2 "Traps" refer to certain metallurgical sites within the steel structure, such as
Inclusions, foreign atoms, dislocations, etc. to which atomic hydrogen may bond. Hydrogen
thus bonded is no longer free to migrate to areas of high stress and contribute to the
initiation of embrittlement fracture. Traps may be of the reversible or non-reversible type.
For further information see Professor Troiano's paper [3].
There are many reasons why a fastener may become embrittled. The total manufacturing process
has to be controlled in such a way that the probability of embrittlement will be reduced to
a minimum. This annex gives examples of procedures by which the probability of hydrogen
embrittlement can be reduced during the manufacturing process for electroplating of
fasteners
Fasteners which have been cold worked hardened to 320 HV or above and are to be
electroplated may benefit from a stress relieving process. This process should be carried
out before application of the cleaning process defined in A.3 The temperature and duration
applicable to the process will vary according to the design, manufacturing and heat
treatment conditions of the parts concemed, and shall be notified to the coater, if the
process is required in accordance with clause 12 Parts with a hardness above 320 HV that
have been machined, ground, cold-formed or cold- straightened subsequent to heat treatment
should be treated according to ISO 9587.
Stress relief may not be desirable in cases where residual stresses are intentionally
introduced, for example, screws which are thread rolled after heat treatment.
Hydrogen absorption of the steel, leading to brittle failure after electroplating, may be
induced by the cleaning process.
Unless otherwise agreed, parts heat-treated or work hardened to a hardness of 320 HV or
above should be cleaned with an inhibited acid, alkaline or mechanical process Immersion
time in the inhibited acid depends on the as-received surface condition and should be of
minimum duration.
NOTE Inhibited acid is an acid to which a suitable inhibitor has been added to reduce
corrosive attack on the steel and absorption of hydrogen.
Parts heat treated or cold worked to a hardness greater than 385 HV or property class 12.9
and above, should not be subjected to acid cleaning treatment. Special pre-treatments are
advisable using non-acidic methods such as dry honing, abrasive blasting or alkali
derusting.
Steel parts should be supplied with a surface which can be prepared for electroplating with
a minimum Immersion time for cleaning.
For fasteners heat-treated or cold-worked to a hardness greater than 365 HV high cathodic efficiency electroplating solutions are advisable.
With increasing hardness, increasing degree of cold working and increasing content of carbon
and/or certain other elements of steel parts, the solubility of hydrogen and therefore the
amount of absorbed hydrogen during an acid cleaning or electroplating process increases. At
the same time, the critical amount of hydrogen which may cause brittle fracture decreases.
The beneficial effect of a baking process after electroplating is removal of hydrogen by
effusion and/or irreversible trapping of hydrogen in the steel.
Parts should be baked within 4h and preferably within an hour of electroplating and before
chromating, to a part temperature of 200 deg *C to 230 deg *C. The maximum temperature
should
take into account the coating material and type of base material. Certain coatings e.g. tin,
and the physical properties of some parts, may be adversely affected by these temperatures.
In such cases, lower temperatures and longer temper durations will be required. This should
be agreed beteen purchaser and supplier.
With increasing coating thickness the difficulty of removing hydrogen increases. The
introduction of an intermediate baking process when the coating is only 2mu*m to 5mu*m thick
may reduce the risk of hydrogen embrittlement.
The user may agree that other conditions for ernbrittlement reduction may be used provided
they can be shown to be effective.
It should not be assumed that the baking recommended will completely prevent hydrogen
embrittlement in all cases.
Alternative baking times and temperatures may be used if they have been shown to be
effective for a part, but parts
should not be baked at a temperature above the temperature at which the parts were
originally tempered. Generally,
lower baking temperatures require longer times at temperature. The chemical composition of
some steels, in
combination with process conditions, may produce a higher susceptibility to hydrogen
embrittlement. Fasteners with
larger diameters are less susceptible than those with small diameters.
At the time of publication of this International Standard it was not considered possible to
give exact baking durations Eight hours is considered a typical example of baking duration.
However, baking durations in the range of 2 h to 24 h at 200 deg * C to 230 deg * C may be
suitable according to the type and size of part, part geometry, mechanical properties.
cleaning processes and electroplating processes used.