Hydrogen Embrittlement ........Do You Take the Risk?
COATING INFORMATION LINK
HYDROGEN EMBRITTLEMENT IN ELECTROPLATED
HIGH TENSILE FASTENER PRODUCTS
FASTENERDATA SUGGESTS THAT FASTENERS AT RISK ARE PRODUCED AND ELECTROPLATED BY THE PRODUCT MANUFACTURER,
AND ARE THEREFORE INSURED BACK TO SOURCE.
The mechanism begins with hydrogen atoms diffusing through the metal fastener or forging wire. When these hydrogen atoms re-combine in minuscule voids of the metal matrix to hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength, up to where it can crack open, in which case it would be called Hydrogen Induced Cracking Hydrogen embrittlement can happen during various manufacturing operations or operational use, anywhere where the metal comes in contact with atomic or molecular hydrogen. Processes which can lead to this include cathodic protection, phosphating, pickling, and electroplating.
Hydrogen embrittlement TECHNICAL EXTRACTS
Hydrogen embrittlement is a major cause of fastener failure. The prevailing thought is that steels with Rockwell hardness above C30 are vulnerable. The phenomenon is well-known although the precise mechanism has eluded extensive research. A number of proposed mechanisms have been proposed, and most have at least some merit. Current thinking is that the susceptibility to hydrogen embrittlement is related directly to the trap population. Generally, hydrogen embrittlement can be described as absorption and adsorption of hydrogen promoting enhanced decohesion of the steel, primarily as an intergranular phenomenon.
Electroplating is a major cause of hydrogen embrittlement. Some hydrogen is generated during the cleaning and pickling cycles, but by far the most significant source is cathodic inefficiency, which is followed by sealing the hydrogen in the parts. Baking is often performed on high strength parts to reduce this risk, and the ASTM, in 1994, issued a specification for baking cycles, as shown below. For the production plater, having to remove the parts from the production line to bake followed by a separate chromating process, is laborious.
How much Baking Do Electroplated fastening Parts need?
EXTRACT FROM ASTM B 850-94
1700 - 1800
49 - 51
Per ASTM B 850-94 "For Steels of actual tensile strenth below
1000 MPa, Heat treatment after plating is not essential."
Hydrogen Embrittlement Extract from British Standards
Electroplating and Hydrogen embrittlement in Metric Steel Fasteners was deleted from BS3382-1961
There are two types of hydrogen embrittlement; firstly the environmental type when it is hydrogen assisted failure due to the supply of hydrogen from the environment, i.e. through corrosion. The second is hydrogen embrittlement where failure is due to various processes used during manufacture.
What is Hydrogen Embrittlement?
When atomic hydrogen enters steel and other alloys it can cause a loss in ductility or load carrying ability or cracking (usually as sub-microscopic cracks), or catastrophic brittle failures at applied stresses well below the yield strength or even normal design strength for the alloys.
This phenomenon often occurs in alloys that show no significant loss in ductility, when measured by conventional tensile strengths, and is frequently referred to as hydrogen induced delayed brittle failure, hydrogen stress cracking or hydrogen embrittlement.
Hydrogen is the smallest atom possible and is the most abundant element in the universe. Two hydrogen atoms combine to form a molecule H2 that is a stable state. For hydrogen to damage steel, it must be in the atomic form, and usually recently produced, called nascent hydrogen. The atom is so small it can enter the structure of steel.
Hydrogen Embrittlement Insertion
The hydrogen can be introduced by various processes; heat treatment, cleaning, pickling, phosphating and electroplating.
Stress corrosion cracking, stress embrittlement, hydrogen embrittlement, and hydrogen-assisted stress corrosion are failure mechanisms which are often viewed as being synonymous, and understandably so because their cause and effect have similarities that outnumber their identifiable differences. Actually only stress corrosion cracking and hydrogen-assisted stress corrosion is corrosion related. However, this is an appropriate time to discuss the other two as well.
So often, with mechanical fasteners, when fatigue failures occur, hydrogen embrittlement is usually blamed. One must, therefore, examine carefully the situation in order to be certain which of the two has caused a failure.
All occur only in fasteners and components, which are stressed in tension and all-cause failure, the actual breaking of the component into two or more pieces. The fracture is delayed. Sometimes it occurs within hours after the tensile load is applied; sometimes not for months, but seldom years. But when it happens, it’s sudden, with no advance warning or any visible signs of imminence. Failures occurring in service are serious and costly, sometimes catastrophic.
Hydrogen Embrittlement in Mechanical Fasteners
Hydrogen embrittlement is associated with fasteners made of carbon and alloy steels. It is usually expected to be a risk for higher tensile fasteners with hardness above 320HV. It is caused by the absorption of atomic hydrogen into the fastener’s surface during the manufacture and processing, particularly during acid pickling and alkaline cleaning prior to plating, and then during actual electroplating.
The deposited metallic coating entraps the hydrogen against the base metal. If the hydrogen is not relieved by a post-baking operation, when load or stress is applied the hydrogen gas migrates towards points of highest stress concentration. Pressure builds until the strength of the base metal is exceeded and minute ruptures occur.
Hydrogen is exceptionally mobile and quickly penetrates into any recently formed cracks, lesions, or material surface discontinuities, which become high-stress areas. Cracks will promulgate through the component surface, weakening the component due to the loss of cross-section area. The failure is usually completed by a ductile fracture. The tougher the material, the more capable it is of resisting the above phenomenon.
The susceptibility of any material to hydrogen embrittlement in a given test is directly related to the characteristics of its trap population. In this instance, trap population relates to the material microstructure, dislocations, carbides, and other elements present in the structure. Such is the effect that interactions can be reversible or irreversible sources. Diffusion is controlled by the rate of escape of hydrogen from the traps; the nature and the density of the traps control the diffusion coefficients.
The greater the hydrogen concentration becomes. The lower the critical stress, or lower the hydrogen concentration, the higher the critical stress at which failure may occur.
Hydrogen embrittlement is non-corrosive related. It is interrelated to high hardness values of the component part. Products having Vickers hardness exceeding HV 320 require special care to reduce the risk of this phenomenon during the plating process or coating procedures.
Some experts feel that hardness exceeding HV 390 is a threshold beyond which further steps to manage hydrogen embrittlement risk are required, often ensuring that acid is not used in the cleaning process; this would be prudent.
The embrittlement process is a random effect, and the De-embrittling process can be regarded similarly.
Hydrogen Embrittlement Fastener Risk Management
Full, stringent and best practice process control is required, from raw material to end product, if one is to limit Hydrogen Embrittlement occurrence.
The amount of hydrogen that may be introduced in, and during the manufacturing process is cumulative. The manufacturer should establish a series of checks to assure that all manufacturing sequences where hydrogen may be potentially introduced are optimised to reduce the production of hydrogen. Lubricants should be monitored to determine they are not used beyond the time period recommended by the lubricant manufacturer.
The vast majority of processing embrittlement risk appears to be attributed to the electroplating process. Before electroplating can take place, parts have to be chemically clean with an active surface. The cleaning process is typically alkaline degreasing followed by acid pickling to remove heat treatment scale, rust, and other oxide films. Acid pickling produces nascent hydrogen, so it is advisable and often mandated in specifications, that this should be kept to a minimum time. Alternatives such as alkaline de-scaling, a slow and expensive process, or mechanical cleaning can be used, and often must be used, for very high tensile components.
Low-temperature heat treatment (baking) is required to REDUCE the risk.
The structure of the coating from an acid solution has a laminar structure, which, due to its lack of porosity, does not allow hydrogen to diffuse from the surface readily. The advantage of a columnar structure, which is given by an alkaline solution, as opposed to laminar, allows the hydrogen to diffuse through the coating.
Some plating specifications note this point and direct that only certain types of plating solutions should be used. The electroplating efficiency of a solution varies with the electrical current density, and in barrel plating, the barrel loading, the rotational speed, contact efficiency, solution temperature, etc. can affect results. So barrel loading is important to reduce hydrogen embrittlement risk.
Thus, the manufacturer should work closely with the electroplater to ensure risk reduction is achieved in prepared steps, for the plating process, and that process care and checks are in place to prevent over or underfilling of plating barrels.
Hydrogen embrittlement mechanisms are thought to be diffusion controlled and thus the effects of time delay before bake are very important.
When high strength fasteners are involved, then no longer than one hour between plating to entry into the bake oven is often mandatory. The transfer time is important and governed by finishing specifications. Large bodies of evidence exist stating that delays exceeding four hours following plating are detrimental to the effectiveness of baking.
Depending upon product types, the baking time will vary from 2 hours for case hardened fasteners to 24 hours for very high tensile or safety critical fasteners.
Higher baking temperatures will increase diffusion rates and should improve de-embrittlement but there are risks of tempering high tensile fasteners and also liquid metal embrittlement occurring, so baking temperatures of between 1800C and 2200C are usually recommended. Lower temperatures and increased baking times are required when this temperature affects the coating or the fastener material.
Hydrogen Embrittlement Reference
Reference documents to consider for the de-embrittlement processes and other useful application information are listed.
BS 7371 – Part 1: 1991 Coating of metal fasteners.
ISO 4042: 2000: Fasteners electroplated coatings.
BS EN ISO 15330: 1999 Preloading test for the detection of hydrogen embrittlement.
BS EN ISO 20898-1: 1999 Mechanical properties of fasteners.
BS EN ISO 20898-2: 1994 Mechanical properties of fasteners.
Because residual and applied stresses are the drivers for hydrogen migration and interaction, it appears that higher strength (higher hardness) fasteners are more sensitive to any delays. The rapid transfer into the baking oven possibly reduces the opportunity for harmful hydrogen to begin its inward migration. It is the prevention of inward migration that will reduce the probability of embrittlement failure.
It is important to note that time at a given temperature should be based on metal temperature (core) of the product being baked.
One great dilemma for the fastener industry is the problem of thread build up after plating. When the end product fails thread gauging, after de-embrittlement, can you rework the components or not? The advice is to discuss the situation with you plating finisher and customer to evaluate the risks. Best is not to try. But you may be faced with a dilemma and here one of the parties will have to assess the situation and make an appropriate decision based on the risk factors.
If fasteners are stripped in an acid solution, extended baking times should be used immediately after the stripping, and prior to re-plating with further post plating de-embrittlement.
Furthermore, the movement by the international standards bodies is not to state baking times, and that the customer should specify his requirement or the finisher should note the best advice and favour longer baking times for complete satisfaction.
Fasteners at Risk, Consider alternative surface coatings.
Here is a listing of some alternative surface coatings and plating methods that can reduce the susceptibility to hydrogen embrittlement:
Dacromet ®. Delta Tone®. Delta Seal®. Xylan®.
De-embrittlement Legal Requirements
The legal position, within the European Union, changed in 1997. Previous to this, a prospective claimant had to prove negligence. Even though like the fastener industry, the surface industry is bound by the requirements of ISO 9002 and QS 9000, where applicable, records are required for seven years minimum and the processor must be able to present a traceable record of events. This in itself does not absolve the supplier from any liability.
The position is that the claimant only needs to prove the part was faulty and unfit for the use, or purpose for which it was supplied in order to claim liability and recompense.
This should motivate responsible suppliers to look for risk-free solutions to these problems.
fastenerdata offer this information as best known practice and will not be held accountable for any liabilities that may result from the information in this document.