September 2009 Archives

When metals are subjected to pickling processes, the metals are dissolved by the acids and hydrogen is generated. The hydrogen is also generated during electrolytic de-greasing, electrolytic pickling, and electro-plating. This hydrogen is occluded (absorbed) by the base metal, especially steel alloys and makes the steel brittle. This phenomenon is the Hydrogen Embrittlement. Parts with hydrogen embrittlement can break after being subjected to loadings. Here, we'll look into some testing methods to see how much hydrogen embrittlement there is on plated steel.

(1) Delta Gage

This is a method developed by Delta Research Co. headed by Mr. Takada of Takada Labs. The method uses steel plates made of alloys susceptible to hydrogen embrittlement, press bent in a constant speed vise, and the vise travel distance to the point of breakage is measured. The distance traveled indicates the degradation of flexibility of the test specimen, in turn indicates the extent of the hydrogen embrittlement. It is also called "Slow press-bent destruction method". Fig. 1 below shows the measurement principle of the Delta Gage method.

In Fig.1., consider that...

then...

For the test specimen: material; SK5 (C0.85%), shape; 9mm x 100mm x 0.8mm thick, hardened at 850 deg.C, tempered at 450 deg.C is used.
The test is performed by placing the flat test specimen between the jaws of the vise, and slowly closing the jaws at a constant speed to bend the test specimen. If any hydrogen occlusion is present in the test specimen, the hydrogen will migrate diffusively towards the area of tensile stress concentration, and the specimen becomes brittle and likely to break in comparison to a specimen with no hydrogen occlusion.

As seen, the Delta Gage method can numerically establish the hydrogen embrittlement rates with easy operations, excellent for factory floors where the plating processes take place.

(8) Standards regarding the baking process

(1) ISO standards

In ISO standards, hydrogen embrittlement relief of electro-galvanized steel is provisioned as follows.

a. Stress relief before plating

Stress relieving heat treatments are necessary for parts made with steel alloys with tensile strength higher than 1000N/mm2 (or hardness shown in note 2) that have been: extensively cold worked, annealed, ground, or subjected to severe machining. (Note 2: Hardness HRC30, HV295, HB280)

b. Heat treatment processes after plating

Post-plating heat treating is necessary for parts that are expected to receive stress and/or static loadings, and for parts that are produced by severe cold working processes. (Table 1) below shows the guideline for heat treating conditions. For parts produced from case-hardened alloys that may be negatively affected by the heat treating guideline, processes with lower temperature at longer durations are required.

(Table 1) Guideline for heat treating steel alloy parts after plating

(2) JIS standards

The JIS standards also defines post-plating heat treatment references as shown in Table 2 below.

(Table 2) Heat treating guideline for stress relieving

(6) Hardness of steel and effects of baking

Effects of baking vary with the hardness of the steel alloy material to be treated. The baking is less effective on harder material, where softer material returning better results. This is said to be contributed not by the hydrogen being purged out of the steel, but by the diffusive migration of the hydrogen from the material's surface region to the inner region, lowering the surface hydrogen concentration to below destructive limits.
For bright zinc plating and cadmium plating, the baking is effective for low hydrogen embrittlement susceptible steel alloy with hardness of HRC40 and below. But not so with steel material with hardness of HRC46 and above.
For material that are carburized, the baking treatment is effective since only the surfaces are hard. It is thought that the occluded hydrogen would heat-diverge into the inner region of the material, and the surface hydrogen content will fall below the destructive limits. However, the hydrogen still remains within the material and may diffusively migrate to regions where stress may be concentrated if any tensile stress is applied.

(7) Summary of Baking Process

Effects of baking processes on plated products are as follows.

	1.	Removal efficiency of the hydrogen occluded during the pre-plating processes is low.
	2.	Baking process is highly effective against the occluded hydrogen during plating processes.
	3.	Effects of baking depends on the hydrogen permeability of the coating. The effects are more pronounced on coatings with coarse-crystalline structure, with pours and cracks.
	4.	Baking is less effective on thick coating layers.
	5.	Baking effectiveness is lower at lower baking temperatures.
	6.	Baking effectiveness is high with steel material with low hydrogen embrittlement susceptibility.
	7.	Post plating sit-out time duration requirement would depend on the thickness of the material. Thin plates are not affected.
	8.	Baking will cause diffusive migration of the hydrogen into the inner region of the material.
	9.	Chromating is to be applied after the baking process. If the baking is applied after the chromate coating process, the chromate coat cracks due to dehydration and the anti-corrosive properties will be degraded.

(2) Plating coat layer and effects of baking

The effects of baking have close relationship with hydrogen permeability of the plating coat layer. Plating coat layers, since being metallic, have a variety of crystalline structure. And this structure is related to hydrogen permeability. Zinc and cadmium are of hexagonal close-packed crystalline structure and of low hydrogen permeability. It is considered, therefore, that dehydrogenation is difficult by baking. For this reason, porous cadmium plating is used for aircraft parts where hydrogen is purged through the pores and cracks. Chrome plating has chrome-specific cracks in the layer that the hydrogen can dissipate, and the baking process is known to be effective for that reason.

In order for the baking process to be effective...

	1.	plating types with pin-holes,
	2.	plating types with cracks and holes,
	3.	plating types with rough coat layers,
	4.	semi-gloss and non-gloss plating

seem to be in use.

(3) Plating layer thickness and effects of baking

When purging the hydrogen from base metal and plating layers, thick plating layer will naturally be less permeable for the hydrogen. Longer baking time will be required for these cases.

(4) Process temperature and effects of baking

The baking process temperature is typically at 190~220 deg.C, but the high temperature may not be suitable for some material that loses hardness or strength by heating. In these cases, the baking time is extended.

(5) Sit-out time after plating

There are platers who set prescribed time between plating and baking such as within 30 minutes, within 2 hours, etc. for example. According to experiments with sit-out times of 1~24 hours, there seems to be little difference for the effects of baking.