Lamellar tearing

When rolled products are used, it is important to consider that when there are some loads in its through-thickness axles, there is a risk of lamellar tearing. The purpose of this article is to give an overview of this effect, in order to consider some parameters that every designer should take into account when a vessel/offshore platform is being designed.

Lamellar tearing is a separation in the parent or base metal caused by through-thickness strains. Such strains are induced primarily by weld metal shrinkage under conditions of high restraint. This tearing always lies within the base metal, usually just outside the visible heat-affected zone (HAZ).

How to distinguish lamellar tearing from other forms of cracking?

The characteristic fibrous or woody appearance of the fracture surface together with the terraced profile and location within the base material characterize a lamellar tearing. They appear as a straight line in the base metal parallel to the direction of rolling of the plate.

Design factors.

There are some design factors which has influence in the internal resistance of the joint. They are:

-          Weld orientation: T and corner joints have a high risk of lamellar tearing because their weld fusion boundary remains parallel to the direction of rolling.

-          Joint Restraint: Size, balance and distribution of the weld has influence in lamellar tearing strength.  For example, weld throat bigger than necessary increase the weld shrinkage strains. Indeed joint designs with large single-sided welds cause unsymmetric strains. However, double sided welds reduce and balance the shrinkage strains and, as consequence, the risk of lamellar tearing.

-          Component restraint: Structural elements with thick and/or curved plates have more risk of lamellar tearing than thin and/or flat plates.

-          Weld metal strength: Sometimes, when the yield point of the weld metal is higher than that of the base metal, the concentration of stresses in this last one increase the risk of lamellar tearing.

Material factors.

There are a lot of parameters than can be considered in evaluating the risk of lamellar tearing. But we are going to focuse our attention on the most important.

-          Exogenous inclusions: usually consist of ladle refractory, ingot scum, or slag that is occasionally trapped in the ingot during solidification.

-          Indigenous inclusions: Their origin is because chemical reaction of elements in the steel or elements added in the steel, usually during deoxidation. So, its quantity depends on a lot of factors, such as: hot working temperature, deoxidation process, chemical composition etc. Manganese sulfides, oxides, alumina and silicates are the dominant inclusions which reduce the throught-thickness strength. So, to reduce the risk of lamellar tearing it is important to reduce the percentage of sulfur and manganese. Ferrite-pearlite banding in the steel matrix has also been reported to cause both initation and propagation of lamellar tears.

Different connection risk.

In a same time of connections, the welding used has influence in the throught thickness strength. For example, in a T joint single or double side full penetration welding have the greater risk of lamellar tearing that a simple fillet or a partial penetration. This lamellar risk in T joints can be represented in the next sketch.

                In corner joints, lamellar tearing risk also depends on the connection and the welding used.

FREQUENCY OF LAMELLAR TEARING PROBLEMS.

                Frequency of lamellar tearing is not high in ship building. This value is increased slightly when the thickness of plating is above 20 mm, when there are high stresses in their through thickness axle “Z”.

                However, this risk is much higher in mobile or fixed offshore platforms due to the large number of highly restrained T and cruciform joints. Frequency of lamellar tearing in offshore platforms could be 1/15-1/20.

                On the other hand, through-thickness strength in high strength steel used to be much lower than in mild steel. This is only valid for rolled products, lamellar tearing is rare in forgings and this problem does not exist in castings.

WELDING TECHNIQUES.

-          Weld size and shape: Using the minimum weld size as soon as possible is good to avoid lamellar tearing problems, as well as increasing the length of the leg on the base plate, which help to distribute the strains over more of the base metal.

-          Buttering: This method consists in adding one or more layers on the plate in where lamellar tearing risk is going to be reduced. This new metal has a low yield strength. The purpose of this steel property is to accommodate the weld shrinkage strains by spreading them more uniformly in this new metal, as well as displacing the heat affected area away from the susceptible parent metal. In this method, some values has to be considered – see sketch -.

CLASSIFICATION SOCIETIES RULES.

In an overview to Bureau Veritas Rules, it was noted some time ago that there were some different requirements about lamellar tearing depending of the rules applied.

In case of Offshore rules May 2014 Edition PtB, Ch3, Sec2, [4.1.1].

“Where, as a result of service or residual stresses, tensile loads are induced normal to the steel plates, the use of Z steel plates is to be specified and adequate structural detail design and special welding techniques may need to be implemented to minimise through thickness loads and weld shrinkage strains.

Note 1 :   Tensile loads induced normal to the steel plates can happen in the following cases: e.g. intersection of tubular elements with large fillet or full penetration welds, or cruciform type joints of heavy elements, or connections of the flange type, or reinforcements of cut-outs and penetrations in way of structural elements subject to large tensile stresses.”

And in Offshore rules May 2014 Edition PtD, Ch1, Sec3, [2.4.2].

“The maximum allowable stress through thickness  is 50% of the allowable yield stress. For Z-grade plates as defined in the Ship Rules, a maximum stress of 75% of allowable yield stress can be accepted as through thickness  stress.”

As can be seen the requirements to avoid lamellar tearing problems in Offshore Industry is quite severe, because of the big number of connections with high risk of lamellar tearing in this industry.

However, in vessels rules requirements avoid lamellar tearing were not so clear. Sometimes in case of plating with stresses in its through thickness axle, “Z” grade was required for thick plates. But there were not any clear requirements. At present, Bureau Veritas has adopted some requirements of Offshore Rules to try to avoid this problem.

In case of Steel Ships July.2014 Edition PtB, Ch4, Sec 1 [2.7.1]:

 “Where normal tensile loads induce out-of-plane stress greater than 0,5 Ry in steel plates:

• for plates with t < 15 mm:
  ultrasonic testing is to be performed
• for plates with t ≥ 15 mm:
  Z-quality steel is to be used or ultrasonic testing is to be performed

in order to prevent laminar tearing.”