Hydrogen embrittlement in high-strength spring wire

What hydrogen embrittlement is

Hydrogen embrittlement is a loss of ductility caused by hydrogen absorbed into a high strength steel. The hydrogen collects at grain boundaries, inclusions and stress concentrations, and it can start a crack that grows slowly under a load the part would normally carry. The failure is often delayed: the spring passes a test, sits in storage or in service, and breaks hours or days later.

The fracture is usually brittle, with little visible deformation, so it is easy to confuse with an overload or a material defect. The history matters: if the part was pickled, electroplated or cleaned in an acid bath, and it broke below its design load, hydrogen is a candidate.

Hydrogen can also enter during service, for example from corrosion or cathodic protection. That is a slower route, but it matters for a spring in a wet, galvanic environment. The process controls at the plater address the manufacturing route, not the whole life of the part.

Which springs are at risk

The risk rises with the strength and hardness of the steel. A high carbon music wire at spring temper, a cold drawn stainless spring wire and a high strength alloy spring are all more exposed than a soft, low carbon part.

The stress state matters as well. A spring that is formed to a tight radius, or that carries a high residual stress from coiling, has less margin. A part that is already close to its allowable stress is less able to tolerate a hydrogen-induced crack.

Geometry adds another factor. A sharp change of section, a tool mark, a scratch or a cut end concentrates stress and gives a crack a place to start. A good surface and a generous radius reduce the risk but do not remove the need for process control.

Where hydrogen enters

The most common route is electroplating. In an acid or alkaline plating bath, hydrogen can be generated at the cathode and absorbed into the steel. Zinc, nickel and zinc-nickel plating are all used on springs, and all can introduce hydrogen if the process is not controlled.

Pickling and acid cleaning are another route. A spring that is descaled or cleaned in an acid bath before plating can absorb hydrogen before the plating starts. The acid concentration, the time and the temperature all affect the amount absorbed.

Corrosion in service is a third route. A galvanic couple, a scratch through a coating or a wet environment can generate hydrogen at the surface. That is why a coated spring still needs a sound surface, not just a bake at the end of the line.

Baking: the control

Baking is a controlled heat treatment after plating that allows hydrogen to diffuse out of the steel. It is not a guarantee. The effectiveness depends on the temperature, the time, the delay between plating and baking, the coating thickness and the strength of the steel. A thick coating can slow the diffusion and trap hydrogen at the interface.

The delay matters. Hydrogen moves slowly through steel, and a part that sits for a day before baking may have hydrogen deeper in the section than one baked immediately. The process specification should state the maximum delay, not just the bake time.

The temperature has to be high enough to move the hydrogen and low enough not to soften the spring. For a spring temper wire, that window can be narrow. Agree the bake schedule with the plater and the wire supplier, and record the furnace chart.

Coating routes with lower risk

Mechanical plating deposits a zinc or zinc alloy coating by tumbling the parts with glass beads and a metal powder. It avoids the electrolytic hydrogen generation of electroplating, so it is often preferred for high strength springs. It still needs a specification for thickness, adhesion and corrosion performance.

Zinc flake coatings are applied from a dispersion and cured. They avoid electroplating and can give good corrosion resistance, but the cure temperature and the coating thickness need to suit the spring. A thick coating can bridge a coil gap and change the solid height.

Passivation is the usual finish for stainless springs. It removes free iron and helps the oxide layer form, and it does not involve the same hydrogen path as electroplating. It does not add a sacrificial coating, so it is not a substitute for zinc on a carbon steel spring.

What to put in the purchase order

The purchase order should name the coating, the standard, the thickness and the bake requirement. It should also state the maximum delay between plating and baking, the acceptance test and the records that must come with the batch. Without those, the plater will use a house process that may or may not fit a high strength spring.

• Wire grade, temper and tensile strength range
• Coating type, standard and minimum thickness
• Bake temperature, time and maximum delay after plating
• Cleaning and pickling controls before plating
• Acceptance test and sample size
• Records: bath log, furnace chart and coating thickness
• Handling and packing to avoid scratches after coating

Testing and verification

A bend test or a sustained load test can show whether a batch is embrittled, but the result depends on the test method. A sustained load test holds the part at a stress below its yield and checks for failure. It is a screen, not a proof of infinite life.

For a critical spring, agree the test with the supplier and the plater before the first batch. Ask whether the test is on the finished part or on a test coupon, and whether the coupon is from the same heat and the same coating batch. A coupon that is not representative proves very little.

Keep the process records with the test result. If a later batch fails, the records show whether the bake, the coating thickness or the cleaning step changed. That is the difference between a correction and a guess.

A practical route

Start by asking whether the spring needs a coating. A stainless spring in a dry environment may need only passivation. If a carbon steel spring needs a coating, choose a route with a lower hydrogen risk where the corrosion duty allows, and control the bake where electroplating is the only option.

Then write the process into the purchase order, not just the drawing. The drawing says what the part must be. The purchase order says how the coating and the bake are controlled, and what evidence comes with the batch.