ft30075e 电镀除氢标准

technical standard

technical note

Platings Steels

Hydrogen embrittlement of steels

French key words revêtement, traitement thermique, dégazage, fragilisation Document for internal use, but may be distributed externally for technical or commercial reasons.

author :JP. Caffarel

Reproduction forbidden

Schneider Electric Industries SAS

FT30 075(E) ind. AA

January 2003

1/2

Caution - Printed copy only for consultation: please check the release index before use.

Purpose

Provide information concerning hydrogen embrittlement of steels with high mechanical resistance.

Experts to consult

Technical Standardisation and Coordination

Modifications

This note cancels and replaces:

- FI-38 085 (Telemecanique standard, SME)- T.19560.2 (Merlin Gerin standard, volume19 clt 5).

Content

1. Hydrogen embrittlement.............................................................22. Examples of solutions. (2)

2.1. Stress relief annealing...................................................................22.2. Cleaning operations......................................................................22.3. Degassing treatment (2)

3. References (2)

FT30 075(E) ind. AA 2/2

Hydrogen embrittlement of steels

Caution - Printed copy only for consultation: please check the release index before use.

1. Hydrogen embrittlement

Hydrogen penetrates and spreads in steels by diffusion. The result is a deterioration of the mechanical properties of the steel: reduction in its ductility or in its capacity to transmit forces that may generate cracks or fractures under the

effect of a very small stress, less than that of the elastic limit of the material.

This phenomenon of hydrogen embrittlement must particularly be taken into account in the case of parts:-with a high tensile strength (Rm > 1000 N/mm 2

),-or with considerable hardness (HV > 350),-or having undergone surface hardening,

-but there are many exceptions and precautions must be taken for steels with a resistance between 700 and 1000

N/mm 2

.

Hydrogen can be introduced in steel during certain processes such as degreasing, bright dipping,

phosphatization, electroplating (e.g. zinc). During machining operations, certain inappropriate lubricating products may introduce hydrogen.

2. Examples of solutions

There are many reasons for the embrittlement of

mechanical parts. For this reason, all the manufacturing processes must be monitored to minimise the probability of hydrogen embrittlement.

A certain number of treatments such as:-stress relief annealing before electroplating,- cleaning operations,- degassing treatment,

reduce the probability of hydrogen embrittlement.

2.1. Stress relief annealing

Annealing is performed for parts with a tensile strength

> 1000 N/mm 2

to eliminate stress in parts prior to electroplating.

The stress relief annealing conditions in table 1 (as per ISO 2081) are given as a guideline, except for surface hardened parts. Different conditions combining shorter durations with higher temperatures can be used if their effectiveness has been proved. The treatment must be performed prior to preparation or cleaning in aqueous solution.

Table 1

Tensile strength N/mm 2Temperature °C Duration h Rm ≤ 1050--1050 < Rm ≤ 1450190 to 22011450 < Rm ≤ 1800190 to 22018Rm > 1800

190 to 220

24

Surface hardened parts must be brought to a temperature of between 130 and 150 °C.

2.2. Cleaning operations

The cleaning process may cause the absorption of

hydrogen by the steel and result in a cleavage fracture after electroplating.

Unless otherwise specified, heat treated or cold hammered parts with a hardness in excess of 320 HV must be cleaned with an inhibited acid (with a suitable inhibitor) or using an alkaline or mechanical process. Immersion time in this acid depends on surface finish and should be minimised.Heat treated and cold hammered parts, with a hardness in excess of 385 HV and fasteners of a class greater than or equal to 12.9 must not undergo cleaning in acid. Preliminary treatments are required using non-acid methods (e.g. dry polishing).

2.3. Degassing treatment

With the increase in hardening and the degree of cold

hammering of steel parts, the amount of hydrogen absorbed during cleaning or electroplating increases.

Degassing after electroplating eliminates hydrogen by diffusion and/or trapping of hydrogen inside the steel.The degassing of a part must be carried out prior to chromating, within a period of less than 4 hours and preferably within the hour following electroplating (see

table 2). The maximum temperature must take into account the plating material and the type of basic material.

Temperature can affect some plating materials (e.g. tin), in which case a lower temperature and a longer time are required.

Table 2

Tensile strength N/mm

2

Temperature °C Duration h Rm ≤ 1050--1050 < Rm ≤ 1450190 to 22081450 < Rm ≤ 1800190 to 22018Rm > 1800

190 to 220

24

Remark:

It is not guaranteed that the recommended degassing totally

prevent hydrogen embrittlement in all cases. Other conditions are possible, with the supplier’s approval, if they have proved their effectiveness for a specific part.

3. References

-ISO 2081 - 1986: Metal coatings. Electroplated coatings of zinc on iron or steel.

-NF EN ISO 4042 - 1999: Fasteners. Electroplated coatings.