The core parts of QRMH mechanism include hook body, tumble piece, and hook basement.
There are two machining process for this core parts, forged steel & steel plate and casting steel. The Material Handling Industry (lifting, rigging, mooring, towin) is moving rapidly to forge solutions. Hence, other alternative technologies such as casting, additive manufacturing are eliminated. You will find the reasons below:
The main difference between forging and other alternative steel processing technology is that the forging is mechanically advanced after solidification. Thanks to this mechanical performance, inner steel structure is fine, stringy & aligned, creating a strong structured net/mesh.
For example. If you look through an electron microscope at forged and casting parts, you will find the difference shown below:
With this grain flow in the steel part and considering applications in which the principal applied stresses, forgings offer lot of advantages:
- Superior strength of final part.
- Structural integrity and product reliability.
- Reduced process control and inspection requirements.
- More predictable response to heat treating.
Mooring Hook applications are not different; forged and casting mooring hooks are available in the market. Comparing to forging, DIN 15400 specification showcases the influence of the casting process on the properties of hooks shall be taken into consideration.
The new EN 13001-3-5 will consider both options, however, it points out and highlights main difference. It clearly defines that some material factors within casting hooks reduce the static limit design force as well as their fatigue strength.
Summary: Definitively, if a dependable hook and a safe mooring is our goal, the forged option is the best choice. In addition, the high-strength properties of the forging process could be used to reduce sectional thickness and overall weight without compromising final part integrity. Certainly, always keep in mind that, the design engineer needs to evaluate all loading characteristics and conditions before a hook is put in use.
- Material Mechanical Property
Comparing “forged Steel” and “casting steel” by strength calculation, it is found that:
- Forged part gives 26% higher tensile strength than the cast parts, benefit from that, F type QRMH can sustain same or higher mooring load than C type, with a smaller and more compact size.
- Forged part hasa 37% higher fatigue strength and resultsin a factor of six times longer fatigue life，which gives F type hook longer life than C type in same condition.
- Forged part hasa 58% reduction in area when pulled to failure. The castingpart only hasa 6% reduction in area. Which means there would be much greater deformation before failure in a forged part.