Environmental extremes and steel lifting
01 October 2019
Understanding how cold temperatures and marine environments impact the safety of steel lifting equipment.
All lifting equipment is affected by the environment at all stages of its lifetime. Two extreme environments we face in the lifting and material handling industry are cold weather and marine environments. Just in the United States, more than a third of the population live in counties directly on the shoreline, and if you take the population residing within 100 miles of the coast that number doubles. Corrosion is a challenge in costal and marine environments as well as in industrial plants and buildings with high humidity.
Freezing temperatures are a factor in more than half of the states in the U.S. and all of Canada. Consequently, both environments become an important aspect to consider when designing, manufacturing, procuring and using lifting and material handling equipment. Founded in Sweden more than 250 years ago, Gunnebo Industries has designed and manufactured lifting equipment that is used on everything from offshore platforms in the Gulf of Mexico to fishing vessels operating in Arctic climates. The company has built leading know-how in combating the challenges of lifting in extreme environments.
What do manufacturers consider cold temperature? Once the ambient temperature drops below freezing (32° F or 0° C), equipment will be affected. Today’s equipment is commonly designed to work down to -4° F (-20° C), and equipment specifically designed for cold weather may even be designed to work in -40° F (-40° C). Below -40° F (-40° C) there is a significant risk of parts of the system not being designed for this environment as there are many components in a lifting system that are affected in cold temperature: fluids, structural or load-bearing steel, electronics, hydraulics, engines and the like. Consequently, it’s always recommended to consult the manufacturer of each individual component or machine to assure you are taking the right precautions and avoid accidents. This article focuses on steel lifting accessories. On top of considering the properties of the equipment there is also the human factor with serious risk of injury or even death if even part of the skin is unprotected.
The main challenges for any material in extreme cold temperatures are durability, strength, toughness and brittleness as the properties of the material change when the temperature drops. Lifting equipment is designed to elongate before it breaks, acting as a safety feature as the operator can see that the product is being incorrectly used and can stop the operation before a catastrophic failure. However, as the temperature drops, steel passes through something called “Ductile – Brittle Temperature Transition.” As shown in Diagram 1 below, this transition is where the steel becomes more brittle.
The transition temperature is determined by a standardized Charpy Impact test which measures the energy required to break a sample of the steel at the design temperature. The transition temperature is important as once a material is cooled below the transition temperature, it has a much greater tendency to shatter on impact instead of bending or deforming. As the steel passes the transition temperature and becomes more brittle the way it will react to a force from a load changes and consequently the way it fractures also shifts. Diagram 2 shows the differences in fracture types, with (a) and (c) showing the extremes and (b) showing a typical fracture of ductile steel under normal conditions.
For the manufacturer, there are several ways of combating the challenges of cold temperature. It starts with the raw material design. The three main factors in the steel design are grain size, carbon content and alloy contents. By decreasing the grain size and carbon content – as well as adding alloys into the material such as nickel, vanadium and manganese – the manufacturer can create a steel that has a low hardness, a high ductility and is resistant against embrittlement in cold temperatures. The production process is also important since the forging, any welding and heat treatment all affect the final characteristics of the product. Certain stainless steels (e.g. 316SS) are also an option – although more costly – as they don’t go through a ductile-brittle transition due to the nature of their crystalline structure.
For the person procuring and using the lifting equipment there are also precautions to take:
Make sure that you have the right equipment.
- There are multiple factors that affect the steel’s capability to handle cold temperature. Make sure to enquire about the how the manufacturer designed and produced the equipment to handle the temperature.
- Think of the system! The lifting gear might be useful down to -40° F (-40° C) but is the winch, block, crane, rope?
- Don’t exceed the limits – such as operational temperature range and capacity – specified by the manufacturer. If you do so you assume the liability since manufacturer recommendation was not followed.
Consider where the products might end up. A sling for an offshore container made in Louisiana might end up in the North Sea.
Decrease velocity of lift if possible. Never shock-load.
Utilize a qualified engineer to determine if the lifting capacities need to be de-rated for your application and/or assure that the system is suitable in cold environments. Lifting operations are always hazardous and cold temperatures makes it even more so, for both man and machine, so making sure you have the right equipment and correct operation practices is critical.
When operating in coastal areas and around or in water there are three main challenges: fatigue, corrosion and brittleness. All three factors affect the steel and the interaction amongst them affects the service life of the product.
The risks and effects of brittleness, as well as the precautions, in a marine environment are like those experienced in cold temperatures but the sources of the brittleness are different. In a marine environment, brittleness comes from a process, called hydrogen embrittlement. Hydrogen can enter the steel in the steel-making process, through the production process (for example if exposed to acids), through the chemical reaction that takes place during corrosion and through absorption from certain environments. As a result, hydrogen embrittlement might cause unexpected brittle fractures at loads below the stated working load limit. This in turn can result in catastrophic failures of the lifting equipment and potential damage to person and property.
Corrosion is the most obvious challenge in a humid or marine environment. Corrosion is a naturally occurring chemical process where processed steel turns into a more chemically stable form such as an oxide. The corrosion rate is affected by multiple factors such as temperature, depth, currents, salinity, humidity levels, wear and pollutants. ISO 9223 defines levels of corrosion rate from C1 (very low, for example air-conditioned warehouse and certain deserts) to CX (extreme, for example offshore structures in the splash zone). A C1 corrosion rate is less than 0.1 µm/yr (0.004 mils/yr) on a galvanized steel plate and a CX is more than 8.4 µm/yr (0.33 mils/yr), see Table 1 on page 24.
On an uncoated steel surface, the corrosion rate along with wear and erosion can be more than what is outlined in Table 1 on page 24 and will be clearly visible. Apart from changing to a stainless steel, hot dip galvanization (HDG) is one of the most effective ways of protecting a steel product against corrosion. It’s a more economical alternative. Paint, including zinc rich paints, is typically only functional if the coating is complete and intact, and with the wear and tear of lifting operations they seldom are. For this reason, many products at risk of corroding are hot dip galvanized. HDG also provides additional benefits such as protecting the steel through acting as anode corroding in place of the load bearing steel, easier inspections as there is no risk for corrosion under the coating as well as high impact and wear resistance – whilst still retaining a higher capacity compared to an equivalent product in stainless steel.
The corrosion that takes place in marine environments, in combination with the forces that act on the lifting equipment in operations, can cause stress corrosion cracking. The corrosion takes place in microscopic cracks which can make the lifting equipment seem in good condition on the outside but then cause an unexpected and catastrophic failure. Alloys, which are used in most lifting equipment and especially in conjunction with a high hardness (generally defined as above 41 HRC), are at a higher risk for stress corrosion. One way of reducing the risk for stress corrosion is to choose lifting equipment with a lower hardness. Another factor increasing the risk of stress corrosion and cracking is residual stress in the products. In the lifting industry a common inspection technique is proof testing. Proof testing is by far the best way to detect subsurface cracks that are not detectable through a visual or magnetic particle inspection. However, during a proof test the lifting equipment is subjected to significant stresses that remain in the material after the load is released and can increase the risk of stress corrosion. One way to combat this for the manufacturer is to add a stress-relieving process, an additional heat treatment after the proof test. However, this is not feasible on certain products and for annual inspections that include proof testing. A good practice is to choose lifting equipment that has been proof tested from the manufacturer – which would show any subsurface cracks – and then refrain from proof testing in the annual inspection as subsurface cracks are unlikely to occur after the manufacturing process.
The third factor affecting steel in a marine environment is fatigue, which is the weakening of material caused by the repeated action of applying loads. All steel products have some microscopic discontinuity, essentially a crack so small that it cannot be seen with the naked eye. As loads are lifted and unloaded the stresses on the material causes these small cracks to grow and finally reach a critical size where the material fractures. The number of stress cycles (or lifts) a material can handle at a determined load is called the fatigue life. The heavier the load in relation to the maximum capacity of the lifting equipment the fewer cycles the material can handle.
For example, at the breaking load the material can handle half a cycle (loading but not unloading) and at the working load limit they might be able to handle tens of thousands of cycles. Fatigue life is affected by a number of factors such as surface defects – issues with paint/coating, notches, cracks, gauges – corrosiveness of the environment, properties of the steel (such as high ductility and high impact toughness which can be determined by a Charpy-V test) and the applied load in correlation to breaking strength of the steel. Fatigue resistance will be one of the limiting factors when determining service life of any lifting equipment. Fatiguing will always be a risk in any lifting application, and it can occur at lower load than WLL, especially in harsh environments.
When using lifting equipment in marine environments, coastal areas or other humid environments, consider these workplace safety tips:
- Hardness and Charpy impact values are more important than the grade of the material. A higher-grade material can be more suitable for a marine environment than a lower grade steel, depending on how the product has been designed and the production process.
- Protection of the products (for example hot dip galvanizing) makes them safer and significantly improves product lifetime and long-term cost.
- Steel lifting equipment might look similar (e.g. master links). The difference is in the details.
- Stay safe. Always consider where the products are going to be used, and where they might end up. If operations take place close to the maximum capacity (working load limit) of the product, consider going up one size to increase the overall strength of the equipment. Always follow manufacturers’ recommendations (both for liability and safety concerns). Adapt your inspection frequency and procedures for the environment and application. Do not modify (for example galvanize or weld) any lifting equipment that is not designed for it as it can have a severe effect on the mechanical properties the steel as well as transferring the liability to the person or company doing the modification.
Lifting, even if it’s just 50 pounds, is risky. Extreme cold temperatures and marine environments make it even more so. Always take proper consideration in the selection, procurement and use of lifting equipment. Always consult the OEM if you are unsure about the properties or the suitability of the equipment.
This article was compiled by the Gunnebo Industries/Crosby Group team of Özkan Kosmaz, R&D manager, Metallurgical Engineer, MSc.; Ernie Lutter, president and engineering vice president, P.E., MEngME; and Felix Nyberg, global product manager, industrial engineer, MSc.