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DRAFT HY80 Improvements (primary sources will be added at conclusion of final draft).

HY80

Is a high-tensile, high yield strength, low alloy steel. It was developed for use in Naval applications, specifically the development of pressure hulls for the US nuclear submarine program and is still currently used in many Naval applications.

The "HY" steels are designed to possess a high yield strength (strength in resisting permanent plastic deformation). HY80 is accompanied by HY100 and HY130 with each of the 80, 100 and 130 referring to their yield strength in ksi (80,000 psi, 100,000 psi and 130,000 psi). HY80 and HY100 are both weldable grades; whereas, the HY130 is generally considered unweldable.

Metallurgy

HY80 steel is a member of the low carbon, low alloy family of steels and is hardenable. The weldability of the steel is good, though it does come with a set of challenges due to the carbon and alloy content. The carbon content can range from 0.12 to 0.20 wt% with an overall alloy content of up to 8 wt%. It is also used extensively in military/Navy applications with large thick plate sections that add to the potential weldability problems e.g. ease of heat treatment and residual stresses in thick plate. The primary objective during the development of the HY grades of steel was to create a class of steels that provide excellent yield strength and overall toughness, which is accomplished in part by quenching and tempering. The steel is first heat treated at 900 degrees celsius to austenitize the material before it is quenched. The rapid cooling of the quenching process produces a very hard microstructure in the form of martensite. Martensite is not desirable and thus it is necessary for the material to be tempered at approximately 650 degrees celsius to reduce the overall hardness and form tempered martensite/bainite.

The final microstructure of the weldement will be directly related to the composition of the material and the thermal cycle(s) it has endured, which will vary across the base material, Heat Affected Zone (HAZ) and Fusion Zone (FZ). The microstructure of the material will directly correlate to the mechanical properties, weldability and service life/performance of the material/weldment. Alloying elements, weld procedures and weldment design all need to be coordinated and considered when looking to use HY80 steel.

Alloying Elements

The alloy content will vary slightly according the thickness of the plate material. Thicker plate will be more restrictive in its compositional alloy ranges due to the added weldability challenges created by enhanced stress concentrations in connective joints.

Importance of Key Alloying Elements:

Carbon - Controls the peak hardness of the material and is an austenite stabilizer, which is necessary for martensite formation. HY80 is prone to the formation of martensite and martensite's peak hardness is dependent on its carbon content. HY80 is a FCC material that allows carbon to more readily diffuse than in FCC materials such as austenitic stainless steel.

Nickel - Adds to toughness and ductility to the HY80 and is also an austenite stabilizer.

Manganese - Cleans impurities in steels (most commonly used to tie up sulfur) and also forms oxides that are necessary for the nucleation of acicular ferrite. Acicular ferrite is desirable in HY80 steels because it promotes excellent yield strength and toughness.

Silicon - Oxide former that serves to clean and provide nucleation points for acicular ferrite.

Chromium - Is a ferrite stabilizer and can combine with carbon to form chromium carbides for increased strength of the material.

Trace Elements:

Antimony, Tin and Arsenic are potentially dangerous elements to have in the compositional makeup due to their ability to form eutectics and suppress local melting temperatures. This is an increasing problem with the increased used of scrap in the making of steel in the Electric Arc Furnace (EAF) process.

Existing Table in HY80 to remain.

Weldability

HY80 steel can be welded without incident provided proper precautions are taken to avoid potential weldability issues. The fact that HY80 is a hardenable steel raises concerns over the formation of untempered martensite in both the Fusion Zone (FZ) and the heat affected zone (HAZ). The process of welding can create steep temperature gradients and rapid cooling that are necessary for the formation of untempered martensite, so precautions must be taken to avoid this. Further complicating the weldability issue is the general application of HY80 steels in thick plate or large weldments for naval use. These thick plates, large weldments and rigorous service environment all pose additional risks due to both intrinsic and extrinsic stress concentration at the weld joint.

HIC or HAC - hydrogen induced or hydrogen assisted cracking is a real weldability concern that must be addressed in HY80 steels. HAC/HIC can occur in either the Fusion Zone or the Heat Affected Zone. As mentioned previously the HAZ and FZ are both susceptible to the formation of martensite and thus are at risk for HAC/HIC. The Fusion Zone HIC/HAC can be addressed with the use of a proper filler metal, while the HAZ HIC/HAC must be addressed with preheat and weld procedures. Low hydrogen practice is always recommended when welding on HY80 steels.

It is not possible to autogenous weld HY80 due to the formation of untempered martensite. Use of filler metals is required to introduce alloying materials that serve to form oxides that promote the nucleation of acicular ferrite. The HAZ is still a concern that must be addressed with proper preheat and weld procedures to control the cooling rates. Slow cooling rates can be as detrimental and rapid cooling rates in the HAZ. Rapid cooling will form untempered martensite; however, very slow cooling rates caused by high preheat or a combination of preheat and high heat input from the weld procedures can create a very brittle martensite due to high carbon concentrations that form in the HAZ.

Welding Filler Metal

Generally, HY80 is welded with an AWS ER100S-1 welding wire. The ER100S-1 has a lower Carbon and Nickel content to assist in the dilutive effect during welding discussed previously.

Distortion and Stress

Given the compositional differences between the base material and the composite zone of the weld it is reasonable to expect that there will be potential distortion due to non-uniform expansion and contraction. This mechanical effect can cause residual stresses that can lead to a variety of failures immediately after the weld or in service failures when put under load. In HY80 steels the level of distortion is proportional to the level of weld heat input, the higher the heat input the higher levels of distortion. HY80 has been found to have less in-plane weld shrinkage and less out-of-plane distortion than the common ABS Grade DH-36