What is high-tensile strength steel? Benefits of use　vol.３~Standards System and Usage Precautions~

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Table of Contents

1. About the Standards System for High-Tensile Steel (WEL-TEN)
2. Is high-tensile steel difficult to handle??
3. Summary

1.About the Standards System for High-Tensile Steel (WEL-TEN)

Here, we will focus on the standards of Nippon Steel’s WEL-TEN® series, which is a representative brand of high-tensile steel plates.

Nippon Steel’s high-tensile steel, the WEL-TEN® series, includes 55 kg steel and above.

50 kg steel is already widely used and is therefore not included.

60 kg steel is classified as the WEL-TEN590 series because it is 60 kg, not 58 kg. There is also 70 kg steel, which falls between 60 kg and 80 kg. When listed in a table, it looks like this.

Standards System for WEL-TEN® (High-Tensile Steel)	base	economic efficiency	non-tempered	Cr-based medium alloy	preheat reduction type
55 kg steel	WEL-TEN540
60 kg steel	WEL-TEN590	WEL-TEN590E	WEL-TEN590RE		WEL-TEN590EX
70 kg steel	WEL-TEN690	WEL-TEN690E	WEL-TEN690RE
80 kg steel	WEN-TEN780	WEL-TEN780E	WEL-TEN780RE	WEL-TEN780C	WEL-TEN780EX
100 kg steel	WEL-TEN950	WEL-TEN950E	WEL-TEN950RE

There are a few other details, but we will omit them this time. As you can see, the scope is quite extensive.There are many types of 80 kg steel, particularly WEL-TEN780, WEL-TEN780E, WEL-TEN780C, and WEL-TEN780EX.”

The Nippon Steel catalog indicates that WEL-TEN780 is the base, but rather, WEL-TEN780 is a high-grade steel that emphasizes toughness. A considerable amount of Ni (Nickel) is added. Ni is an expensive element, but it is effective in enhancing the toughness of steel materials. It is applied to marine structures, among others.

WEL-TEN780E is a general-purpose 80 kg steel commonly used in construction machinery and other applications.WEL-TEN780C contains Cr (Chromium) and was developed for use in spherical tanks, among other applications. However, since the maximum thickness of WEL-TEN780E is 60 mm, it is also used as a thicker 80 kg steel to cover greater thicknesses.

WEL-TEN780EX contains Cu (Copper), and it is a preheat reduction type 80 kg steel that minimizes the occurrence of welding cracks by utilizing the increased strength provided by the copper.

Additionally, there is also 80 kg steel manufactured by hot rolling and untreated, known as WEL-TEN780RE, which is applied to construction machinery and other uses.

There are many options, and it can be difficult to accurately choose the optimal steel type. If you have any questions, please feel free to contact us at Kumagai.

2.Is high-tensile steel difficult to handle??

As previously mentioned, the toughness values of high-tensile steel are more strictly regulated than those of mild steel, but are there any other things to watch out for?

In the case of bending processes, it requires extra force for high-tensile steel because the deformation occurs near the tensile strength, exceeding the yield strength. This phenomenon is unavoidable. In some cases, it is possible to perform the processing while the material is in a soft state before heat treatment and then carry out the heat treatment afterward to achieve the desired strength.

Toughness sometimes only becomes favorable after heat treatment, so it is also important to consider whether the base material can withstand the processing before heat treatment. If it cracks during preliminary bending, everything is lost.

Additionally, due to the high yield strength, the elastic range at the start of bending becomes wider, resulting in a larger springback effect, where the bent shape slightly returns to its original form.

Although the amount of springback can be calculated, there are variations between steel materials, so adjustments are necessary initially.

welding of high-tensile strength steel

Improvements have been made regarding the welding of high-tensile strength steel.

There was a concern that high-tensile steel, which contains a large number of alloying elements, was prone to cracking during welding. Therefore, it was necessary to preheat the steel plate before welding (this is called preheating) to prevent rapid cooling after welding.

However, in TMCP steel, the amount of alloying elements has been reduced, significantly alleviating this concern. Even for steels other than TMCP, how to reduce crack-prone components and make the steel more resistant to cracking is a key area of expertise for steel manufacturers, so please consult with them.

It is also necessary to select welding materials with high strength that match the base material.Usually, materials slightly stronger than the base material are used. Since the welded part is relatively small in the overall structure, if that part is softer, deformation will concentrate there, leading to failure.

To increase the strength of the weld, it is necessary to increase the alloy content in the weld metal. However, this makes it more likely for “delayed cracking” to occur. When both the steel plate and the weld metal have a high alloy content, preheating the steel plate before welding is necessary to prevent this.Indicators of welding cracks include the Carbon Equivalent (Ceq) and the Weld Crack Sensitivity Index (Pcm).

Some steel standards regulate these factors. Whether cracking occurs in the weld area is greatly influenced by the amount of hydrogen that enters during welding. Therefore, various welding materials are available that control the amount of hydrogen.

High-tensile steel manufactured by quenching and tempering is designed to have minimal characteristic changes when subjected to localized rapid heating and cooling. However, inappropriate treatment can lead to softening or reduced toughness, so caution is necessary.

Additionally, heating to temperatures higher than the tempering temperature can cause softening, making hot working unsuitable. Even during warm working, it is important to pay attention to the temperature.When stress-relief annealing (SR) is carried out after processing or welding to reduce residual stresses, caution is necessary. Especially for steels above 80 kg, there are types that can experience reduced toughness, known as ‘SR embrittlement,’ so it is essential to confirm and specify this in advance.

High-tensile steel has high strength, but its deflection is no different from that of ordinary steel.Because of its high strength, the plate thickness can be reduced, but reducing the thickness increases the amount of deflection. Therefore, to achieve rigidity, it is necessary to devise reinforcements and other measures.Applications where it can bend like a fishing rod without issue are limited to thick plates.

Another important consideration is fatigue strength.

In applications subjected to repeated loads, small cracks may occur at some point. Once these small cracks form, they can cause stress concentrations, leading to the cracks spreading, similar to the saying ‘a small leak can sink a great ship.’

High-tensile steel has a higher strength for crack initiation compared to mild steel. However, it is known that there is almost no difference in the weld joints between mild steel and high-tensile steel. When using high-tensile steel, it is common to design with higher strength on the member, and combined with the residual stress left from welding, these small cracks tend to occur more easily.

As a method to mitigate this, some companies offer the sale and rental of devices called UIT (Ultrasonic Impact Treatment) that increase the fatigue strength of weld joints.

This can enhance the fatigue performance of high-tensile steel to approach its inherent capabilities.

3.Summary

While writing all of this out, many people may think that high-tensile steel is more difficult to handle compared to mild steel. However, with the correct usage and familiarity, it becomes a very useful material. Please feel free to contact our company.

We are a specialized trading company based in Japan, dealing in high-tensile steel and other special steels.

Please feel free to contact us.

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