Sheffield Forgemasters is the UK’s only steel plant capable of producing bespoke heavy steel castings and forged ingot products for the world’s most demanding applications. Sheffield Forgemasters hold N-stamp accreditation from AMSE and produces nuclear island components for both civil and defense markets. Understanding and controlling alloy chemistry during steel making and subsequent casting, forging and heat treatment are crucial for dictating the mechanical properties and in service behaviour of high integrity, safety critical components. This talk will highlight the role of chemistry on steel behaviour and detail aspects of how this is controlled. It will also reveal case studies detailing how chemistry can be manipulated to meet customer needs, and show current research and development programs running to address future market requirements.