16 Inventories

		2017	2016
	US$m		US$m
Raw materials and purchased components		648		470
Consumable stores		875		840
Work in progress		1,082		970
Finished goods and goods for resale		1,027		800
		3,632		3,080
Comprising:
Expected to be used within one year		3,472		2,937
Expected to be used after more than one year		160		143
		3,632		3,080

During 2017, the Group recognised a net inventory write-back of US$7 million. This comprised a US$81 million write-back of amounts previously written down due to an increase in realisable values, offset by inventory write offs of US$74 million. In 2016, inventory write-downs, net of reversals, amounted to US$51 million and in 2015 US$192 million.

At 31 December 2017, US$611 million (2016: US$564 million) of inventories were pledged as security for liabilities.