Comparing density stainless steel and titanium for subsea laser welding

Abstract

Modelling material mass and volume behavior is critical in subsea engineering, particularly in the design and operation of offshore oil and gas facilities, and deep-sea communication systems. With an increasing number of exploration projects being carried out in ultra-deep water, the choice between austenitic stainless steels and titanium is becoming a critical decision based on the materials’ thermophysical properties. This technical note reviews and examines the density of stainless steel, specifically the 300 series of stainless steels, and how it stacks up against Grade 5 titanium, and also presents the impact of this when carrying out the underwater laser welding process. The iron metal density and the resulting hydrostatic interaction within the weld pool will be modeled in relation to the structural integrity of the weld as well as buoyancy management of the structure. The general objective of this technical note is to provide sufficient information to Tier-1 marine suppliers and manufacturers to enhance material performance and to ease logistics in the global foreign trade market.

The subsea engineering pain point: The conflict between ballast requirements and structural fatigue

Our engineers are often struggling with the subsea manifold design and ROV chassis design, where there is a need for heavy ballast to counteract the buoyancy caused by the current, and at the same time strong structural design with minimum material usage to avoid fatigue failure. The other major issue in the offshore oil and gas industry is the unpredictable buoyancy shift during the assembly of heavy components. If the designers don’t have the density of the stainless steel material at their disposal, the “dry weight,” which is calculated on the design drawings, is always different from the “wet weight” of the component (real weight when the component is submerged). This discrepancy can sometimes be fatal to the heavy machinery used to lift the assembled components, and sometimes the lift equipment fails to provide the exact precision required in high pressure under water conditions.
Subsea laser welding also has additional challenges due to the rapid quenching effect of the water environment. The iron mass density of stainless steel is also quite different from that of titanium. The weld pool solidification behaviour under pressure is different for each material. This is an issue for those manufacturers who use the high iron density stainless steels, such as 316L, in order to provide maximum corrosion resistance. It is also important to consider the mass-moment of inertia of an assembly in order to avoid any stability issues in deep water ( depths greater than 3,000m). This guide provides further information on the need for quantitative material characterisation to avoid such problems.

The fundamental role of density stainless in subsea structural equilibrium

To design for the structural stability of all marine applications, the designer must begin with knowledge of the relative density of the seabed, or some other layer of the ocean, to which they are interested in designing their component. As a first approximation, we take the density of stainless steel of the 300-series austenitic alloys. Carbon steel, for example, has an iron metal density of approximately 7.87 g/cm³, whereas stainless steels, because of the high nickel and chromium contents that must be present for adequate corrosion resistance, have a greater mass, which, together with their useful properties, act to compensate for other more adverse properties of the metals.
The density of stainless steel is approximately 8.00 g/cm³, 8000 kg/m³ for a standard 304 or 316L alloy. This is higher than the aluminum density of 2.70 g/cm³ used for near-surface components in many typical aerospace applications and ensures that the negative buoyancy required for a subsea template to remain anchored to the seabed is satisfied. Also, the density of stainless steel must be correctly specified in foreign trade documents in order to meet the requirements of North American clients. The density of stainless steel is approximately 0.289 lb/in3. Although a 50-ton subsea module for an offshore oil project may seem to be a large, straightforward quantity, an error in the second decimal place of the density of stainless (which is hardly noticeable in calculations of the size and weight of structural components in many applications) can amount to hundreds of kilograms, a serious amount of tonnage when attempting to get a heavy subsea module to its destination in an offshore project and a real challenge when arranging transoceanic shipment.

Crystallographic comparison: density stainless vs. titanium alloys

Material density is largely dependent on the structure of atoms in that particular material. Stainless steel is face-centered cubic (FCC) and has a high atomic packing factor. This makes it denser and gives it the stainless values seen. Titanium Grade 5, also known as Ti-6Al-4V, has a hexagonal close-packed (HCP) structure.
Although titanium is well known for its strong composition, its density is 4.43 g/cm³. When comparing the two to each other, the density of stainless steel is 8.00 g/cm3, which is almost twice as heavy as the titanium. High-density stainless steel, however, is a good material for a gravity-based foundation because the heavy mass ensures the stability of the equipment. In contrast, low-density titanium is better suited to moving components on ROVs, as energy consumption is lower.
METALLURGICAL INTRODUCTION: The iron density in the stainless alloy is what makes up the mass. Although the alloy elements, such as molybdenum, have a higher atomic weight than iron, the macroscopic density across the stainless series remains relatively constant. Only slight variations exist, such as the density of 303 stainless steel (8.03 g/cm³), which allows for specific mass adjustments for turned and machined products in the case of fasteners.

Managing hydrostatic pressure through density stainless material selection

In subsea laser welding, one variable that affects joint quality is hydrostatic pressure. The laser produces a vapor capillary known as a keyhole, which has to be in a stable condition to avoid porosity, and the internal pressure of the keyhole is balanced by the pressure due to the weight of the solidifying metal column. The pressure P is given by:

where ρ is the density of stainless steel. Due to the high density of stainless steel, the liquid metal exerts a heavy downward force on the keyhole. This high hydrostatic pressure is also useful in deep-sea environments, as it helps to prevent the creation of vapor bubbles, which would otherwise form porosity.
Low aluminum alloy (2.70 g/cm³) tends to have a volatile keyhole, which collapses more easily than high-density stainless at higher speeds. Also, it provides a self-stabilizing effect in the melt pool. Thus, with some adjustment of focusing the fiber laser, the material can be welded efficiently at subsea, over a wide range of speeds and focal positions. But the major attribute is that higher laser power density is required in order to dynamically support the weight of the high-density material, in order to create a fully molten track. It’s vital to know the exact density of stainless steel in order to precisely calculate the laser focal head position and travelling speed in order to ensure full weld penetration without sagging of the weld bead.

Laser-matter interaction: How density stainless influences the subsea melt pool

A steam pocket is created on the surface of a stainless steel plate when it is hit with a high-energy laser beam while submerged. The laser material interaction is dependent on the thermal diffusivity of the material, which is inversely proportional to the density of the material. Stainless steel has high density.
The formula for thermal diffusivity(α)is:

The high density of stainless steel in the denominator results in a thermal diffusivity that is lower than in aluminum but higher than many of the titanium alloys. The reason density stainless is effective in the weld zone is that it retains the heat. However, the water being applied to the weld has a profound effect as a massive heat sink.
Several factors can influence the weld quality, particularly in titanium welding, where the lower density and lower thermal conductivity can lead to a “heat buildup” that can cause grain growth and embrittlement. Stainless steel welds benefit from a predictable density stainless melt pool, provided the power source is adjusted to give a controlled cooling rate. A good appreciation of the thermal profile is required to satisfy the ever more demanding requirements of the international maritime authorities, and to facilitate the compliance of the subsea products when subject to the rigorous NDT procedures.

Thermal conductivity and density stainless in deep-sea welding dynamics

Deep-sea environments involve temperatures near 4° C and pressures in excess of 300 bar. At these levels, the physical constants of the material are the only reliable metrics for process control. The density of the stainless steel dictates the “volumetric heat capacity” of the joint.
Since the density of stainless is higher than that of titanium, more energy is required to heat a single cubic millimeter to the melting temperature. However, the high mass density of iron in the molten stainless steel means that the liquid metal will tend to stay within the joint boundaries and is not washed away by the large forces of the surrounding water. On a 316L subsea sensor, precision laser welding works very well, since the density of stainless steel means that thin-walled sensor housings do not deform or suffer excessive distortion under the intense heating that occurs in a very small area at the laser focus.
What is the density of al in relation to common engineering materials? This is a question that is often asked when practicing engineers move from the aerospace business to the subsea business. With that information in mind, you’ll understand why aluminum is not frequently used as a material in high-pressure subsea housings. Low aluminium density means that the material is what is known as ‘buoyant’ and therefore not able to satisfy the required compressive strength standards required for such deep water applications, and that’s why high-density stainless materials are used instead. In subsea, more mass means more stability, and stainless steels have the necessary density to provide the structural base for long-term use of subsea systems.

Corrosion resistance and the localized density stainless profile in saltwater

One of the secondary benefits of the 300-series is the stability of the protective oxide layer. Although Titanium is the most corrosion-resistant metal, stainless steel 316L is more economical and offers a lower density, which is beneficial for larger structures such as subsea risers.

Pitting Corrosion in Saline Environments is a major concern in international trade. Pitting occurs when the chromium oxide layer is penetrated. Weld joints can have an inconsistent stainless profile if the density is affected by porosity or elemental segregation.

By using high-precision laser welding with the lowest possible heat-affected zone (HAZ) we can ensure that the entire weld area has a uniform density of stainless, which means that there is no variation in the electrochemical potential, which means that there will be no unwanted “galvanic cells” that can cause the structural failure of the component. Our customer is an operator who wants to achieve the longest possible lifetime of their components. They know that this will depend on a homogeneous density stainless substrate.

Logistics and foreign trade: Shipping costs related to density stainless products

From a commercial point of view, the density of stainless steels is a major factor in the “Landed Cost” of subsea components. When, for example, a foreign trade company in Asia is shipping 316L manifolds to the North Sea, the weight of the material will dictate the size of the vessel required and thus the freight cost.
Since stainless steel has a density of 8.0 g/cm³, these items are considered to be weight-intensive. A standard 20-foot shipping container will reach its maximum weight capacity of approximately 28,000 kg before it is even 50% full by volume. In contrast, shipping aluminum extrusions has a low density, allowing them to be packed all the way to the ceiling of the container.
Chargeable weight is also a concern for global suppliers. When it comes to air freight for emergency subsea repair parts, the high density of stainless steel means that the actual weight of the parts will always be greater than the chargeable weight, which is based on volumetric weight. Knowing the density of aluminum in lb/in3 as compared with the density of stainless steel in lb/in³ allows the procurement officer to make a quick determination of when a material change to aluminum may be necessary in order to avoid weight restrictions when leaving the port of origin.

Precision and tolerances: Maintaining micrometer-scale accuracy using density stainless alloys

Subsea connectors, fiber-optic repeaters. Our production of subsea connectors and fiber-optic repeaters demands micrometer precision in order to obtain a watertight seal. The density of the stainless material has an impact on the “damping” caused by the vibrations during the CNC-machining and laser-cutting processes.

Some material choices will contain high iron metal density. Although hard to believe, high iron density material will give better frequency absorption to high-frequency spindle vibrations. Essentially, the machinist can obtain a smoother surface finish on a dense material (stainless steel) than can be obtained on a lighter material (titanium or aluminum). This is due to the effect of the “mass” of the material, which is creating a stabilizing inertia on the cutting tool that will prevent any unwanted vibrations caused by the tool chatter.

Weldability of dense stainless steel. The density has a bearing on the thermal expansion behaviour of the components during welding. A dense material will generally have a higher thermal mass and will expand and contract in a more controlled fashion. This aspect of material behaviour is important in the design of subsea components that require interference fits to be achieved with a “zero-tolerance”, which means that there can be no adjustment to the size of the components and that the components must fit together without any adjustment whatever. In such cases, the consistency of the density of the stainless material is the engineer’s only design tool.

Economic trade-offs: Analyzing density stainless versus titanium for high-depth deployment

The choice between the density of stainless steel or titanium is then a matter of budget. Titanium is obviously a much better material in technical terms for our deep-sea ROVs, low density and high strength being all advantages. But at the same time, the price is 5 to 10 times higher than for the 316L stainless steel.

In many static applications where weight is not a concern and actually can be a cost savings, high-density stainless steel is a very cost-effective solution. The global subsea hardware market is still largely driven by dense stainless steel due to the balance between weight, strength, and cost. For foreign trade suppliers of subsea hardware, being able to quote the various densities of stainless (such as the density of 17-4ph or the iron density of some lower-grade stainless) as well as the specific 303 stainless steel density of components such as fasteners and connectors, can be an important factor in gaining credibility with foreign engineering companies.

Thinking about the future of Subsea 4.0, where robotic laser welding automation will perform seabed repairs, it is easy to forget that the fundamental physics of the material we design against has not changed. Whether it’s calculating the ballast needed for a new riser or determining the optimal laser parameters for a seabed repair, the calculation is the same – density of the stainless steel is a constant in our subsea design thinking.

Read More： Is Stainless Steel Maagnetic? Impact on Laser Cutting Accuracy

References

1. Callister, W. D. (2014). Materials Science and Engineering: An Introduction. Wiley. (Crystallographic and mass derivation of the density stainless FCC lattice).
2. Bäuerle, D. (2011). Laser Processing and Chemistry. Springer. (Thermodynamic interactions and the role of fe density in laser-matter coupling).
3. Davis, J. R. (2003). Handbook of Materials for Medical and Subsea Devices. ASM International. (Specific data on the 303 stainless steel density and its series).
4. Steen, W. M. (2010). Laser Material Processing. Springer. (Fluid dynamics of melt pools and the impact of hydrostatic pressure based on density stainless).
5. International Maritime Organization (IMO). Guidelines for the Selection of Materials for Subsea Infrastructure. (Standards for density stainless in marine environments).
6. https://www.marinetechnologynews.com/news/subsea-autonomous-welding-robot-657229
7. https://mii.com.om/advanced-underwater-welding-techniques-ensuring-safety-and-efficiency-in-marine-repairs/

Frequently Asked Questions (FAQs)

1. What is the standard density stainless for subsea applications?
   In the subsea world, 316L stainless is the typical material used. The density of stainless is approximately 8.00 g/cm³. For comparison, the iron metal density of carbon steel is 7.87 g/cm³.
2. Why is density stainless preferred over titanium for seabed templates?
   Because these are weight-sensitive structures, to be stable in current, they will need a large area of seafloor. High-density stainless steel has more self-weight than titanium.
3. How does density stainless affect underwater laser welding?
   The high-density stainless helps to increase the hydrostatic pressure of the melt pool, giving greater stability of the laser keyhole and a porosity reduction, particularly in the high-pressure conditions of the deep-sea environment.
4. Can I substitute aluminum for stainless steel in subsea housings?
   Not usually. Low aluminum alloy density is 2.70 g/cm³, and that is too buoyant. This material does not have the compressive strength of the high-density stainless steel needed to withstand the pressure of the deep ocean.
5. – Change density stainless measurements from metric to imperial for shipping.
   There is metric density stainless at 8.00 g/cm³, which converts to 8.00 g/cm³ x 0.0361 = 0.289 lb/in³. Accurate conversion is key to determining the weight of the tonnage of all of the global maritime shipments.