High strength,Age Hardening Low-Alloy Steel Plate for Offshore Platforms and Hull PIate
Authors: Money K.L. , Shelton C.H. , Rydrean P.P. 
Published: October 2008

A new 0.9Ni-0.6Cr-0.2Mb-l.2Cu0.04Cb steel (IN-787) has exhibited marine corrosion behavior equal to or better than several other low alloy steels to which it was compared. Carbon steel (AISI 1010), hot rolled Cu-Si steel (ASTM A242), a quenched and tempered Cr-Si-Mb steel (ASTM A514), a normalized Si steel (ASTM A537) and a quenched and tempered Si steel were included in the study. The general corrosion, stress corrosion cracking and simulated piling corrosion behavior in a sheltered channel and in the aggressive splash and spray zone, where the specimens were directly exposed to severe wave action, was evaluated for these several steels over a one year test period. The corrosion behavior of this new alloy is much better in the marine atmosphere than the other steels by virtue of the development of a very tight adherent rust film.

The mechanical strength properties and toughness characteristics, both at ambient and sub-zero temperatures, of this steel are presented. Evaluation results are covered for metal thicknesses from 1-1/4" to 7" from commercial heats in the water quenched and aged condition. Based upon the results attained, the IN-787 offers promise as a high strength steel for hull plate and off-shore platform applications, especially where extreme fracture toughness under conditions of severe restraint or to low temperatures is required, combined with excellent weld ability under adverse conditions.

Introduction

Marine corrosion resistance and good mechanical properties are but two of the parameters which must be considered for selection of a material of construction for off-shore platforms or ship hull plate. One reference has stated(l) that the bulk of steel to be used in off-shore platform construction must be very high grade material with good low temperature impact properties, high tensile strength and good welding characteristics. Also steels considered for platform or ship construction should have good sea water corrosion resistance and resistance to stress corrosion cracking, even under conditions of possible hydrogen absorption due to cathodic protection.

In this paper a description is given of the mechanical properties, both at ambient and sub-zero temperatures, of a newly developed steel designated IN-787. Initial results of continuing corrosion tests in natural marine environments are also presented.

Material and Procedure of Marine Corrosion Tests

The chemical compositions of the steels included in this study are listed in Table I. Specimens of these alloys are exposed in the marine atmosphere at Kure Beach, North Carolina and quiet and low velocity sea water at Wrightsville Beach, North Carolina. Piling strip specimens exposed in the sheltered channel are 8 to 10 feet long and extend from the splash and spray Zone to the bottom mud through the tidal and quiet sea water zones. Three-foot long sections of IN-787 and carbon steel are also exposed in the aggressive splash and spray zone on a fishing pier at Wrightsville Beach, N. C., where the specimens are subjected to severe wave action. All specimens were sandblasted, weighed (where applicable) and measured prior to exposure.

Details of the base plate and welded stress corrosion specimens are given in Table II.