Forest Products Notes on...

Case Hardening of Timber

Case hardening is an often misunderstood subject in the timber trade.

It is defined as:

a condition of stress and set in dry timber characterised by compressive stress in the outer layers and tensile stresses in the inner layers. An imbalance of stresses.

It is important to remember that:

· Wood shrinks once below Fibre Saturation Point (FSP).

· Wood dries from the surface inwards – so the outer regions of a piece of wood can become appreciably drier (and therefore shrink more) before the inner regions.

(The following diagrams show the stresses developing in the cross-section of a piece of wood as the drying process proceeds. C indicates a region under compressive stress. T indicates a region under tensile stress).

During the drying process a sequence of events may occur:

1) The surface regions dry to a moisture content below FSP before the inner regions .... but the surface regions cannot shrink because they are restrained by the still swollen inner regions. SO THE SURFACE REGIONS GO INTO TENSION AND THE INNER REGIONS ARE PUT INTO COMPRESSION:

Tension (T) and compression (C) regions in drying timber

Now if the drying conditions are not too severe this causes no problem, because these stresses are relieved once the inner regions start to dry and shrink.

But if drying is carried out too rapidly…

2) The surface regions may dry out in their stretched condition.

3) When the inner regions (only now) start to dry, they attempt to shrink, but they can’t - they are restrained by the (“too large”) outer regions. A stress reversal occurs and “case hardened” timber has sadly been produced:-

Case hardened timber in cross section

Case hardened timber will cup on re-sawing causing problems for the processor.

If noticed, case hardening can be rectified by steaming in the kiln: the humidity of the air in the kiln is greatly increased so that the surface regions gain moisture and swell. Now the stresses will disappear.

However, if the surface regions get too moist they attempt to swell too much! Another stress reversal occurs and another defect is produced – this is termed “reverse case hardening”:-

Reverse case hardened timber in cross section

To avoid these problems of case hardening it is important to check for the development of stresses in the timber as it is drying: read about how to monitor the stresses developing in the drying stock on the NEXT PAGE: Avoiding Case Hardening by Monitoring the Drying of Timber...

Tension (T) and compression (C) regions in drying timber
Case hardened timber in cross section
Reverse case hardened timber in cross section
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The various defects that can arise due to the inherent anisotropy of wood are described on the PREVIOUS PAGE: see  Drying Defects in Sawn Timber to learn how warping, twist etc. develop and what should be done to avoid these value-reducing defects...

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