Mark Seward, 49, had only worked for AGD Equipment for 16 days when the accident happened, Warwick Crown Court was told.
He was leak testing a hydraulic cylinder at the company's site in Stratford-upon-Avon. The cylinder cracked under pressure, exploding and sending metal fragments flying. One of these hit the employee in the head.
The Health and Safety Executive's (HSE) investigation found that AGD Equipment had not told Seward the safe working pressure for the cylinder.
The task was not adequately supervised and the company had not erected protective screens to stop staff being injured by projectiles.
AGD Equipment pleaded guilty to breaching regs 12(1) of the Provision and Use of Work Equipment Regulations and 3(1) of the Management of Health and Safety at Work Regulations, for failing to risk assess the task.
It also admitted breaching s 3(1) of the Health and Safety at Work Act as other people were allowed to wander into the test area on site.
The company was fined £800,000 plus £28,711 costs.
HSE inspector Neil Ward said: "It is a company's obligation to provide a safe system of work for leak testing. This will include protecting people from flying fragments and high-pressure oil leaks, as well as providing thorough training in how to carry out the work safely."