Forging cool-down rate impacts mooring performance

Large-scale metal forging is a complex, and far less understood, process than other metal treatment techniques. During forging the metal is heated and shaped by compressive force, refining the grain structure which, in turn, improves mechanical performance, i.e. strength, ductility, and toughness.

Deepwater mooring components must provide high levels of strength and toughness, as well as outstanding fatigue behavior over many years. While the latter relies heavily on established computer modeling techniques, the factors governing strength and toughness are less clearly defined.

At present, forged and cast steel subsea mooring components and accessories are needed to meet relevant chain guides/specifications. Unfortunately, these requirements do not provide a consistent and accurate representation of toughness throughout larger geometry forgings, but more of a generic value. In some circumstances, this fails to give a true indication of the actual operational performance or fatigue resistance of the connector or mooring accessory.

Project goals

The overall aim of the two-year research project was to understand what happens to the metal’s microstructure during the forging process for a large, 480-mm (18.9-in.) billet typically used in a mooring connector, and in so doing:

• Determine the fracture toughness Charpy/ CTOD (Crack Tip Open Displacement) relationship for samples with different microstructures
• Predict toughness values in using a heat transfer model
• Find the best representative sampling location for the forged component
• Determine the optimum heat treatment process, forging process and steel composition to produce the best forged material.

The project was in three parts: the effects of chemical composition, evaluation of test sample locations, and quenching/rate of cooling to validate a heat transfer model.

First Subsea connector during forging.

Changes in chemical composition of the steel were found to influence the material’s microstructure; increasing levels of nickel and manganese showed a direct correlation to higher strength and toughness.

Traditionally, metals testing for large billets in offshore projects involves sampling a third radius from the surface at the end of the bar. Determining the cooling rate profile through the cross-section allowed alternative sampling positions to be evaluated. This showed that end of the bar, sampling is not representative of the material as whole, while samples taken one diameter from the end of the bar, and the middle of the bar, are more representative. A global Heat Transfer (HT) model was developed to predict the temperature in different parts of the bar during cooling.

Findings and results

The quenching trial showed that the metal’s hardness, strength, toughness, ductility and grain size are dependent upon rate of cooling. The HT model was used to predict toughness for 34CrNiMo6 grade steel. Samples were heated to 8,500°C (15,332°F), and cooled at rates from 0.10°C/s (32.18°F/s) to 200°C/s (392°F/s).

Toughness increased with increased cooling rate, for example, 0.50°C/s (32.9°F/s) gives a toughness value of 50 Joules. This has been attributed to differences in microstructure observed as parts of the bar cool at different rates. Analysis also showed that rapid quenching from austenite produces martensite, and that high levels of martensite are needed for increased hardness and toughness.

The First Subsea/IMMPETUS project has highlighted the effect of microstructure in terms of toughness related to different rates of cooling, and the importance of sampling position in achieving representative and consistent toughness values for large diameter billets. In addition, it has allowed First Subsea to determine the optimum forging and heat treatment process conditions needed to give more consistent mechanical properties throughout the length of its subsea mooring connectors and accessories.