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High Yield Strength Steels

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Welding-HY-Steels refers to processing a family of steels that owe their name to High Yield Strength.

This was the main property designed into the materials as they were developed in the form of thick plates.

That occurred in the sixties of the past century, at the request of the US Navy, mainly for fabrication of ship hulls and submarines.

Within this class, that includes HY-80, HY-100, HY-130 and HY-180, which are high strength and toughness, quenched and tempered, martensitic steels.

The single materials are identified by the numbers that represent their yield strength, expressed in ksi (kilo or 1000 pounds per square inch).

Precautions for Welding-HY-Steels

Welding-HY-Steels of these classes of high impact resistance require strict welding conditions to realize their characteristic potential also in the welds and in their heat affected zone (HAZ).

In particular low hydrogen electrodes should be used, with due precautions in keeping them dry and heating them before use in suitable ovens to drive away any moisture.

Otherwise the welds may become prone to Hydrogen Induced Cracking (HIC).

Furthermore Welding-HY-Steels should be performed with minimal heat input, in small stringers without weaving.

The lower yield steels of this class are welded by SMAW with low hydrogen electrodes of Type E-10018 or E-11018.

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The higher yield ones are best welded by GTAW and Plasma arc welding with special filler metals of extremely low impurity levels (oxygen, hydrogen, nitrogen, carbon, sulfur and phosphorus).

For applications requiring Welding-HY-Steels for service exposure to temperatures from 0 to (minus) -195 °C, the ferritic steels with high nickel contents are typically used.

Such applications include storage tanks for liquefied hydrocarbon vapors (gases) and structures or machinery designed for use in cold regions.

These steels utilize the effect of nickel content in reducing the ductile to brittle transition temperature, thereby improving toughness at low temperatures.

Double normalized and tempered 9% nickel steel is covered by ASTM A 353, and quenched and tempered 8% and 9% nickel steels are covered by ASTM A 553 (types I and II).

For quenched and tempered material, the minimum lateral expansion in Charpy V-notch impact tests is specified at 0.38 mm.

These steels remain ductile at the lowest testing temperatures.

The 5% Ni steel retains relatively high fracture toughness at -162 °C and the 9% Ni steel retains relatively high fracture toughness at -196 °C.

These temperatures are the lowest at which these steels may be used, but Welding-HY-Steels requires to develop and follow very strictly suitable welding conditions.

The 5% Ni alloy steels for low-temperature service include HY-130 and ASTM A 645.

For steel purchased according to ASTM A 645 minimum Charpy V-notch impact requirements for 25 mm plate are established at -170 °C for hardened, tempered, and reversion-annealed plate.

Preheating is required for the thicker plates to slow down the cooling rate after welding.

This helps in avoiding HAZ cracking and weld cracking, reducing the amount of untempered martensite, provides the conditions for hydrogen to escape, and limits residual stresses.

Preheating of naval steels of these classes (with a specified minimum yield strength ranging from 80 to 130 ksi) while Welding-HY-Steels is often required to overcome susceptibility to Hydrogen Induced Cracking in the weld heat affected zone (HAZ).

See our page on Hydrogen Embrittlement.

This form of cracking occurs especially in high strength steels that have the potential to form high-carbon twinned martensite when the following conditions are simultaneously present:

  1. a source of dissolved hydrogen;
  2. a susceptible (martensitic) microstructure;
  3. high residual tensile stress;
  4. a temperature range that does not allow significant solid-state diffusion of hydrogen from the steel; and
  5. a time delay following welding that allows hydrogen to accumulate at internal flaws in the steel.

Preheating, interpass temperature control and post weld heating operations, individually or in combination, essentially reduce the dissolved hydrogen content.

This occurs by allowing the hydrogen to escape by diffusion from the steel while also allowing transformation of the weld metal.

More importantly the adjacent HAZ (Heat Affected Zone) becomes a less susceptible microstructure that might reduce the peak residual tensile stress as well.

Nevertheless, preheat, interpass temperature control and post-soak temperature control during Welding-HY-Steels

  • are quite expensive,
  • add to welder discomfort
  • reduce the overall productivity.

Procedures should follow consumable supplier recommendations and be qualified by passing successfully all destructive tests required.

It was soon realized that the strict conformance to precise practices, while achieving acceptable properties in Welding-HY-Steels, resulted in low productivity and high fabrication costs, especially for large structures.

Therefore renewed efforts, again initiated by the requirements of the US Navy, resulted in the development of a new family of materials called High Strength Low Alloy (HSLA) steels that allowed more economic fabrication because their modified microstructure is more tolerant of less strict welding conditions.

An Article on Welding of Ultra-Thin Steel was published (2) in Issue 163 of Practical Welding Letter for March 2017.
Click on PWL#163.

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